I'm using the latest linux kernel mainline code,
with the default arm64 configuration:
make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- defconfig,
and set CONFIG_EXPERT=y, CONFIG_DEBUG_BUGVERBOSE=n,
CONFIG_PANIC_ON_OOPS=y.
Loading the following kernel module will cause kernel panic.
The call stack is as follows:
root@(none):/# /root/insmod hello.ko
[ 6.035003] hello: loading out-of-tree module taints kernel.
[ 6.039129] ------------[ cut here ]------------
[ 6.039287] hello
[ 6.039704] Unexpected kernel BRK exception at EL1
[ 6.040059] Internal error: BRK handler: 00000000f2000800 [#1] PREEMPT SMP
[ 6.040457] Modules linked in: hello(O+)
[ 6.041311] CPU: 0 PID: 50 Comm: insmod Tainted: G O 6.9.1 #8
[ 6.041755] Hardware name: linux,dummy-virt (DT)
[ 6.042238] pstate: 60400005 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
[ 6.042594] pc : buginit+0x18/0x1000 [hello]
[ 6.043601] lr : buginit+0x18/0x1000 [hello]
[ 6.043852] sp : ffff800080533ae0
[ 6.044121] x29: ffff800080533ae0 x28: 0000000000000000 x27: 0000000000000000
[ 6.044523] x26: ffffaba8c4e70510 x25: ffff800080533c30 x24: ffffaba8c4a28a58
[ 6.044961] x23: 0000000000000000 x22: 0000000000000000 x21: ffff3947c0eab3c0
[ 6.045503] x20: ffffaba8c4e3f000 x19: ffffaba846464000 x18: 0000000000000006
[ 6.046124] x17: 0000000000000000 x16: ffffaba8c2492834 x15: 0720072007200720
[ 6.046387] x14: 0720072007200720 x13: ffffaba8c49b27c8 x12: 0000000000000312
[ 6.046829] x11: 0000000000000106 x10: ffffaba8c4a0a7c8 x9 : ffffaba8c49b27c8
[ 6.047293] x8 : 00000000ffffefff x7 : ffffaba8c4a0a7c8 x6 : 80000000fffff000
[ 6.047739] x5 : 0000000000000107 x4 : 0000000000000000 x3 : 0000000000000000
[ 6.047955] x2 : 0000000000000000 x1 : 0000000000000000 x0 : ffff3947c0eab3c0
[ 6.048366] Call trace:
[ 6.048653] buginit+0x18/0x1000 [hello]
[ 6.048922] do_one_initcall+0x80/0x1c8
[ 6.049333] do_init_module+0x60/0x218
[ 6.049475] load_module+0x1ba4/0x1d70
[ 6.049755] __do_sys_init_module+0x198/0x1d0
[ 6.049959] __arm64_sys_init_module+0x1c/0x28
[ 6.050160] invoke_syscall+0x48/0x114
[ 6.050334] el0_svc_common.constprop.0+0x40/0xe0
[ 6.050468] do_el0_svc+0x1c/0x28
[ 6.050635] el0_svc+0x34/0xd8
[ 6.050852] el0t_64_sync_handler+0x120/0x12c
[ 6.051088] el0t_64_sync+0x190/0x194
[ 6.051433] Code: d0ffffe0 910003fd 91000000 9400000b (d4210000)
[ 6.052212] ---[ end trace 0000000000000000 ]---
[ 6.052473] Kernel panic - not syncing: BRK handler: Fatal exception
The kernel module source code is as follows:
```
static int __init buginit(void)
{
WARN(1, "hello\n");
return 0;
}
static void __exit bugexit(void)
{
}
module_init(buginit);
module_exit(bugexit);
MODULE_LICENSE("GPL");
```
When CONFIG_DEBUG_BUGVERBOSE macro is disabled,
the size of "__bug_table" section in hello.ko
is only 6 bytes instead of the expected 8 bytes.
As a result,
mod->num_bugs = sechdrs[i].sh_size / sizeof(struct bug_entry) = 6 / 8 = 0
calculated in module_bug_finalize when the kernel loads ko is incorrect.
When running `WARN()`, the following backtrace is triggered:
module_find_bug() at lib/bug.c
find_bug() at lib/bug.c
__report_bug() at lib/bug.c
report_bug() at lib/bug.c
call_break_hook() at arch/arm64/kernel/debug-monitors.c
brk_handler() at arch/arm64/kernel/debug-monitors.c
It will return -EFAULT because hello.ko's mod->num_bugs is 0.
Finally, the kernel OOPS is triggered.
Add .align 2 to the end of __BUG_ENTRY
to make the object layout generated by the assembly code
consistent with that of the C struct bug_entry.
Fixes: 9fb7410f955f ("arm64/BUG: Use BRK instruction for generic BUG traps")
Signed-off-by: Yuanbin Xie <[email protected]>
Signed-off-by: Jiangfeng Xiao <[email protected]>
---
arch/arm64/include/asm/asm-bug.h | 1 +
1 file changed, 1 insertion(+)
diff --git a/arch/arm64/include/asm/asm-bug.h b/arch/arm64/include/asm/asm-bug.h
index c762038..6e73809 100644
--- a/arch/arm64/include/asm/asm-bug.h
+++ b/arch/arm64/include/asm/asm-bug.h
@@ -28,6 +28,7 @@
14470: .long 14471f - .; \
_BUGVERBOSE_LOCATION(__FILE__, __LINE__) \
.short flags; \
+ .align 2; \
.popsection; \
14471:
#else
--
1.8.5.6
On Fri, May 17, 2024 at 10:13:28PM +0800, Jiangfeng Xiao wrote:
> I'm using the latest linux kernel mainline code,
> with the default arm64 configuration:
> make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- defconfig,
> and set CONFIG_EXPERT=y, CONFIG_DEBUG_BUGVERBOSE=n,
> CONFIG_PANIC_ON_OOPS=y.
>
> Loading the following kernel module will cause kernel panic.
>
> The call stack is as follows:
>
> root@(none):/# /root/insmod hello.ko
> [ 6.035003] hello: loading out-of-tree module taints kernel.
> [ 6.039129] ------------[ cut here ]------------
> [ 6.039287] hello
> [ 6.039704] Unexpected kernel BRK exception at EL1
> [ 6.040059] Internal error: BRK handler: 00000000f2000800 [#1] PREEMPT SMP
> [ 6.040457] Modules linked in: hello(O+)
> [ 6.041311] CPU: 0 PID: 50 Comm: insmod Tainted: G O 6.9.1 #8
> [ 6.041755] Hardware name: linux,dummy-virt (DT)
> [ 6.042238] pstate: 60400005 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
> [ 6.042594] pc : buginit+0x18/0x1000 [hello]
> [ 6.043601] lr : buginit+0x18/0x1000 [hello]
> [ 6.043852] sp : ffff800080533ae0
> [ 6.044121] x29: ffff800080533ae0 x28: 0000000000000000 x27: 0000000000000000
> [ 6.044523] x26: ffffaba8c4e70510 x25: ffff800080533c30 x24: ffffaba8c4a28a58
> [ 6.044961] x23: 0000000000000000 x22: 0000000000000000 x21: ffff3947c0eab3c0
> [ 6.045503] x20: ffffaba8c4e3f000 x19: ffffaba846464000 x18: 0000000000000006
> [ 6.046124] x17: 0000000000000000 x16: ffffaba8c2492834 x15: 0720072007200720
> [ 6.046387] x14: 0720072007200720 x13: ffffaba8c49b27c8 x12: 0000000000000312
> [ 6.046829] x11: 0000000000000106 x10: ffffaba8c4a0a7c8 x9 : ffffaba8c49b27c8
> [ 6.047293] x8 : 00000000ffffefff x7 : ffffaba8c4a0a7c8 x6 : 80000000fffff000
> [ 6.047739] x5 : 0000000000000107 x4 : 0000000000000000 x3 : 0000000000000000
> [ 6.047955] x2 : 0000000000000000 x1 : 0000000000000000 x0 : ffff3947c0eab3c0
> [ 6.048366] Call trace:
> [ 6.048653] buginit+0x18/0x1000 [hello]
> [ 6.048922] do_one_initcall+0x80/0x1c8
> [ 6.049333] do_init_module+0x60/0x218
> [ 6.049475] load_module+0x1ba4/0x1d70
> [ 6.049755] __do_sys_init_module+0x198/0x1d0
> [ 6.049959] __arm64_sys_init_module+0x1c/0x28
> [ 6.050160] invoke_syscall+0x48/0x114
> [ 6.050334] el0_svc_common.constprop.0+0x40/0xe0
> [ 6.050468] do_el0_svc+0x1c/0x28
> [ 6.050635] el0_svc+0x34/0xd8
> [ 6.050852] el0t_64_sync_handler+0x120/0x12c
> [ 6.051088] el0t_64_sync+0x190/0x194
> [ 6.051433] Code: d0ffffe0 910003fd 91000000 9400000b (d4210000)
> [ 6.052212] ---[ end trace 0000000000000000 ]---
> [ 6.052473] Kernel panic - not syncing: BRK handler: Fatal exception
>
> The kernel module source code is as follows:
> ```
>
> static int __init buginit(void)
> {
> WARN(1, "hello\n");
> return 0;
> }
>
> static void __exit bugexit(void)
> {
> }
>
> module_init(buginit);
> module_exit(bugexit);
> MODULE_LICENSE("GPL");
> ```
>
> When CONFIG_DEBUG_BUGVERBOSE macro is disabled,
> the size of "__bug_table" section in hello.ko
> is only 6 bytes instead of the expected 8 bytes.
> As a result,
> mod->num_bugs = sechdrs[i].sh_size / sizeof(struct bug_entry) = 6 / 8 = 0
> calculated in module_bug_finalize when the kernel loads ko is incorrect.
>
> When running `WARN()`, the following backtrace is triggered:
>
> module_find_bug() at lib/bug.c
> find_bug() at lib/bug.c
> __report_bug() at lib/bug.c
> report_bug() at lib/bug.c
> call_break_hook() at arch/arm64/kernel/debug-monitors.c
> brk_handler() at arch/arm64/kernel/debug-monitors.c
>
> It will return -EFAULT because hello.ko's mod->num_bugs is 0.
> Finally, the kernel OOPS is triggered.
>
> Add .align 2 to the end of __BUG_ENTRY
> to make the object layout generated by the assembly code
> consistent with that of the C struct bug_entry.
>
> Fixes: 9fb7410f955f ("arm64/BUG: Use BRK instruction for generic BUG traps")
>
> Signed-off-by: Yuanbin Xie <[email protected]>
> Signed-off-by: Jiangfeng Xiao <[email protected]>
> ---
> arch/arm64/include/asm/asm-bug.h | 1 +
> 1 file changed, 1 insertion(+)
Ouch; sorry about this.
I reckon we should spell out the reason for this a bit more; would you
be happy with the commit message below?
| When CONFIG_DEBUG_BUGVERBOSE=n, we fail to add necessary padding bytes
| to bug_table entries, and as a result the last entry in a bug table will
| be ignored, potentially leading to an unexpected panic(). All prior
| entries in the table will be handled correctly.
|
| The arm64 ABI requires that struct fields of up to 8 bytes are
| naturally-aligned, with padding added within a struct such that struct
| are suitably aligned within arrays.
|
| When CONFIG_DEBUG_BUGVERPOSE=y, the layout of a bug_entry is:
|
| struct bug_entry {
| signed int bug_addr_disp; // 4 bytes
| signed int file_disp; // 4 bytes
| unsigned short line; // 2 bytes
| unsigned short flags; // 2 bytes
| }
|
| ... with 12 bytes total, requiring 4-byte alignment.
|
| When CONFIG_DEBUG_BUGVERBOSE=n, the layout of a bug_entry is:
|
| struct bug_entry {
| signed int bug_addr_disp; // 4 bytes
| unsigned short flags; // 2 bytes
| < implicit padding > // 2 bytes
| }
|
| ... with 8 bytes total, with 6 bytes of data and 2 bytes of trailing
| padding, requiring 4-byte alginment.
|
| When we create a bug_entry in assembly, we align the start of the entry
| to 4 bytes, which implicitly handles padding for any prior entries.
| However, we do not align the end of the entry, and so when
| CONFIG_DEBUG_BUGVERBOSE=n, the final entry lacks the trailing padding
| bytes.
|
| For the main kernel image this is not a problem as find_bug() doesn't
| depend on the trailing padding bytes when searching for entries:
|
| for (bug = __start___bug_table; bug < __stop___bug_table; ++bug)
| if (bugaddr == bug_addr(bug))
| return bug;
|
| However for modules, module_bug_finalize() depends on the trailing
| bytes when calculating the number of entries:
|
| mod->num_bugs = sechdrs[i].sh_size / sizeof(struct bug_entry);
|
| ... and as the last bug_entry lacks the necessary padding bytes, this entry
| will not be counted, e.g. in the case of a single entry:
|
| sechdrs[i].sh_size == 6
| sizeof(struct bug_entry) == 8;
|
| sechdrs[i].sh_size / sizeof(struct bug_entry) == 0;
|
| Consequently module_find_bug() will miss the last bug_entry when it does:
|
| for (i = 0; i < mod->num_bugs; ++i, ++bug)
| if (bugaddr == bug_addr(bug))
| goto out;
|
| ... which can lead to a kenrel panic due to an unhandled bug.
|
| This can be demonstrated with the following module:
|
| static int __init buginit(void)
| {
| WARN(1, "hello\n");
| return 0;
| }
|
| static void __exit bugexit(void)
| {
| }
|
| module_init(buginit);
| module_exit(bugexit);
| MODULE_LICENSE("GPL");
|
| ... which will trigger a kernel panic when loaded:
|
| ------------[ cut here ]------------
| hello
| Unexpected kernel BRK exception at EL1
| Internal error: BRK handler: 00000000f2000800 [#1] PREEMPT SMP
| Modules linked in: hello(O+)
| CPU: 0 PID: 50 Comm: insmod Tainted: G O 6.9.1 #8
| Hardware name: linux,dummy-virt (DT)
| pstate: 60400005 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
| pc : buginit+0x18/0x1000 [hello]
| lr : buginit+0x18/0x1000 [hello]
| sp : ffff800080533ae0
| x29: ffff800080533ae0 x28: 0000000000000000 x27: 0000000000000000
| x26: ffffaba8c4e70510 x25: ffff800080533c30 x24: ffffaba8c4a28a58
| x23: 0000000000000000 x22: 0000000000000000 x21: ffff3947c0eab3c0
| x20: ffffaba8c4e3f000 x19: ffffaba846464000 x18: 0000000000000006
| x17: 0000000000000000 x16: ffffaba8c2492834 x15: 0720072007200720
| x14: 0720072007200720 x13: ffffaba8c49b27c8 x12: 0000000000000312
| x11: 0000000000000106 x10: ffffaba8c4a0a7c8 x9 : ffffaba8c49b27c8
| x8 : 00000000ffffefff x7 : ffffaba8c4a0a7c8 x6 : 80000000fffff000
| x5 : 0000000000000107 x4 : 0000000000000000 x3 : 0000000000000000
| x2 : 0000000000000000 x1 : 0000000000000000 x0 : ffff3947c0eab3c0
| Call trace:
| buginit+0x18/0x1000 [hello]
| do_one_initcall+0x80/0x1c8
| do_init_module+0x60/0x218
| load_module+0x1ba4/0x1d70
| __do_sys_init_module+0x198/0x1d0
| __arm64_sys_init_module+0x1c/0x28
| invoke_syscall+0x48/0x114
| el0_svc_common.constprop.0+0x40/0xe0
| do_el0_svc+0x1c/0x28
| el0_svc+0x34/0xd8
| el0t_64_sync_handler+0x120/0x12c
| el0t_64_sync+0x190/0x194
| Code: d0ffffe0 910003fd 91000000 9400000b (d4210000)
| ---[ end trace 0000000000000000 ]---
| Kernel panic - not syncing: BRK handler: Fatal exception
|
| Fix this by always aligning the end of a bug_entry to 4 bytes, which is
| correct regardless of CONFIG_DEBUG_BUGVERBOSE.
|
| Fixes: 9fb7410f955f ("arm64/BUG: Use BRK instruction for generic BUG traps")
| Signed-off-by: Yuanbin Xie <[email protected]>
| Signed-off-by: Jiangfeng Xiao <[email protected]>
With that:
Reviewed-by: Mark Rutland <[email protected]>
Mark.
>
>
>
> diff --git a/arch/arm64/include/asm/asm-bug.h b/arch/arm64/include/asm/asm-bug.h
> index c762038..6e73809 100644
> --- a/arch/arm64/include/asm/asm-bug.h
> +++ b/arch/arm64/include/asm/asm-bug.h
> @@ -28,6 +28,7 @@
> 14470: .long 14471f - .; \
> _BUGVERBOSE_LOCATION(__FILE__, __LINE__) \
> .short flags; \
> + .align 2; \
> .popsection; \
> 14471:
> #else
> --
> 1.8.5.6
>
>
Hi,
On 2024/5/20 18:33, Mark Rutland wrote:
> On Fri, May 17, 2024 at 10:13:28PM +0800, Jiangfeng Xiao wrote:
>> I'm using the latest linux kernel mainline code,
>> with the default arm64 configuration:
>> make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- defconfig,
>> and set CONFIG_EXPERT=y, CONFIG_DEBUG_BUGVERBOSE=n,
>> CONFIG_PANIC_ON_OOPS=y.
>>
>> Loading the following kernel module will cause kernel panic.
>>
>> The call stack is as follows:
>>
>> root@(none):/# /root/insmod hello.ko
>> [ 6.035003] hello: loading out-of-tree module taints kernel.
>> [ 6.039129] ------------[ cut here ]------------
>> [ 6.039287] hello
>> [ 6.039704] Unexpected kernel BRK exception at EL1
>> [ 6.040059] Internal error: BRK handler: 00000000f2000800 [#1] PREEMPT SMP
>> [ 6.040457] Modules linked in: hello(O+)
>> [ 6.041311] CPU: 0 PID: 50 Comm: insmod Tainted: G O 6.9.1 #8
>> [ 6.041755] Hardware name: linux,dummy-virt (DT)
>> [ 6.042238] pstate: 60400005 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
>> [ 6.042594] pc : buginit+0x18/0x1000 [hello]
>> [ 6.043601] lr : buginit+0x18/0x1000 [hello]
>> [ 6.043852] sp : ffff800080533ae0
>> [ 6.044121] x29: ffff800080533ae0 x28: 0000000000000000 x27: 0000000000000000
>> [ 6.044523] x26: ffffaba8c4e70510 x25: ffff800080533c30 x24: ffffaba8c4a28a58
>> [ 6.044961] x23: 0000000000000000 x22: 0000000000000000 x21: ffff3947c0eab3c0
>> [ 6.045503] x20: ffffaba8c4e3f000 x19: ffffaba846464000 x18: 0000000000000006
>> [ 6.046124] x17: 0000000000000000 x16: ffffaba8c2492834 x15: 0720072007200720
>> [ 6.046387] x14: 0720072007200720 x13: ffffaba8c49b27c8 x12: 0000000000000312
>> [ 6.046829] x11: 0000000000000106 x10: ffffaba8c4a0a7c8 x9 : ffffaba8c49b27c8
>> [ 6.047293] x8 : 00000000ffffefff x7 : ffffaba8c4a0a7c8 x6 : 80000000fffff000
>> [ 6.047739] x5 : 0000000000000107 x4 : 0000000000000000 x3 : 0000000000000000
>> [ 6.047955] x2 : 0000000000000000 x1 : 0000000000000000 x0 : ffff3947c0eab3c0
>> [ 6.048366] Call trace:
>> [ 6.048653] buginit+0x18/0x1000 [hello]
>> [ 6.048922] do_one_initcall+0x80/0x1c8
>> [ 6.049333] do_init_module+0x60/0x218
>> [ 6.049475] load_module+0x1ba4/0x1d70
>> [ 6.049755] __do_sys_init_module+0x198/0x1d0
>> [ 6.049959] __arm64_sys_init_module+0x1c/0x28
>> [ 6.050160] invoke_syscall+0x48/0x114
>> [ 6.050334] el0_svc_common.constprop.0+0x40/0xe0
>> [ 6.050468] do_el0_svc+0x1c/0x28
>> [ 6.050635] el0_svc+0x34/0xd8
>> [ 6.050852] el0t_64_sync_handler+0x120/0x12c
>> [ 6.051088] el0t_64_sync+0x190/0x194
>> [ 6.051433] Code: d0ffffe0 910003fd 91000000 9400000b (d4210000)
>> [ 6.052212] ---[ end trace 0000000000000000 ]---
>> [ 6.052473] Kernel panic - not syncing: BRK handler: Fatal exception
>>
>> The kernel module source code is as follows:
>> ```
>>
>> static int __init buginit(void)
>> {
>> WARN(1, "hello\n");
>> return 0;
>> }
>>
>> static void __exit bugexit(void)
>> {
>> }
>>
>> module_init(buginit);
>> module_exit(bugexit);
>> MODULE_LICENSE("GPL");
>> ```
>>
>> When CONFIG_DEBUG_BUGVERBOSE macro is disabled,
>> the size of "__bug_table" section in hello.ko
>> is only 6 bytes instead of the expected 8 bytes.
>> As a result,
>> mod->num_bugs = sechdrs[i].sh_size / sizeof(struct bug_entry) = 6 / 8 = 0
>> calculated in module_bug_finalize when the kernel loads ko is incorrect.
>>
>> When running `WARN()`, the following backtrace is triggered:
>>
>> module_find_bug() at lib/bug.c
>> find_bug() at lib/bug.c
>> __report_bug() at lib/bug.c
>> report_bug() at lib/bug.c
>> call_break_hook() at arch/arm64/kernel/debug-monitors.c
>> brk_handler() at arch/arm64/kernel/debug-monitors.c
>>
>> It will return -EFAULT because hello.ko's mod->num_bugs is 0.
>> Finally, the kernel OOPS is triggered.
>>
>> Add .align 2 to the end of __BUG_ENTRY
>> to make the object layout generated by the assembly code
>> consistent with that of the C struct bug_entry.
>>
>> Fixes: 9fb7410f955f ("arm64/BUG: Use BRK instruction for generic BUG traps")
>>
>> Signed-off-by: Yuanbin Xie <[email protected]>
>> Signed-off-by: Jiangfeng Xiao <[email protected]>
>> ---
>> arch/arm64/include/asm/asm-bug.h | 1 +
>> 1 file changed, 1 insertion(+)
>
> Ouch; sorry about this.
>
> I reckon we should spell out the reason for this a bit more; would you
> be happy with the commit message below?
>
> | When CONFIG_DEBUG_BUGVERBOSE=n, we fail to add necessary padding bytes
> | to bug_table entries, and as a result the last entry in a bug table will
> | be ignored, potentially leading to an unexpected panic(). All prior
> | entries in the table will be handled correctly.
> |
> | The arm64 ABI requires that struct fields of up to 8 bytes are
> | naturally-aligned, with padding added within a struct such that struct
> | are suitably aligned within arrays.
> |
> | When CONFIG_DEBUG_BUGVERPOSE=y, the layout of a bug_entry is:
> |
> | struct bug_entry {
> | signed int bug_addr_disp; // 4 bytes
> | signed int file_disp; // 4 bytes
> | unsigned short line; // 2 bytes
> | unsigned short flags; // 2 bytes
> | }
> |
> | ... with 12 bytes total, requiring 4-byte alignment.
> |
> | When CONFIG_DEBUG_BUGVERBOSE=n, the layout of a bug_entry is:
> |
> | struct bug_entry {
> | signed int bug_addr_disp; // 4 bytes
> | unsigned short flags; // 2 bytes
> | < implicit padding > // 2 bytes
> | }
> |
> | ... with 8 bytes total, with 6 bytes of data and 2 bytes of trailing
> | padding, requiring 4-byte alginment.
> |
> | When we create a bug_entry in assembly, we align the start of the entry
> | to 4 bytes, which implicitly handles padding for any prior entries.
> | However, we do not align the end of the entry, and so when
> | CONFIG_DEBUG_BUGVERBOSE=n, the final entry lacks the trailing padding
> | bytes.
> |
> | For the main kernel image this is not a problem as find_bug() doesn't
> | depend on the trailing padding bytes when searching for entries:
> |
> | for (bug = __start___bug_table; bug < __stop___bug_table; ++bug)
> | if (bugaddr == bug_addr(bug))
> | return bug;
> |
> | However for modules, module_bug_finalize() depends on the trailing
> | bytes when calculating the number of entries:
> |
> | mod->num_bugs = sechdrs[i].sh_size / sizeof(struct bug_entry);
> |
> | ... and as the last bug_entry lacks the necessary padding bytes, this entry
> | will not be counted, e.g. in the case of a single entry:
> |
> | sechdrs[i].sh_size == 6
> | sizeof(struct bug_entry) == 8;
> |
> | sechdrs[i].sh_size / sizeof(struct bug_entry) == 0;
> |
> | Consequently module_find_bug() will miss the last bug_entry when it does:
> |
> | for (i = 0; i < mod->num_bugs; ++i, ++bug)
> | if (bugaddr == bug_addr(bug))
> | goto out;
> |
> | ... which can lead to a kenrel panic due to an unhandled bug.
> |
> | This can be demonstrated with the following module:
> |
> | static int __init buginit(void)
> | {
> | WARN(1, "hello\n");
> | return 0;
> | }
> |
> | static void __exit bugexit(void)
> | {
> | }
> |
> | module_init(buginit);
> | module_exit(bugexit);
> | MODULE_LICENSE("GPL");
> |
> | ... which will trigger a kernel panic when loaded:
> |
> | ------------[ cut here ]------------
> | hello
> | Unexpected kernel BRK exception at EL1
> | Internal error: BRK handler: 00000000f2000800 [#1] PREEMPT SMP
> | Modules linked in: hello(O+)
> | CPU: 0 PID: 50 Comm: insmod Tainted: G O 6.9.1 #8
> | Hardware name: linux,dummy-virt (DT)
> | pstate: 60400005 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
> | pc : buginit+0x18/0x1000 [hello]
> | lr : buginit+0x18/0x1000 [hello]
> | sp : ffff800080533ae0
> | x29: ffff800080533ae0 x28: 0000000000000000 x27: 0000000000000000
> | x26: ffffaba8c4e70510 x25: ffff800080533c30 x24: ffffaba8c4a28a58
> | x23: 0000000000000000 x22: 0000000000000000 x21: ffff3947c0eab3c0
> | x20: ffffaba8c4e3f000 x19: ffffaba846464000 x18: 0000000000000006
> | x17: 0000000000000000 x16: ffffaba8c2492834 x15: 0720072007200720
> | x14: 0720072007200720 x13: ffffaba8c49b27c8 x12: 0000000000000312
> | x11: 0000000000000106 x10: ffffaba8c4a0a7c8 x9 : ffffaba8c49b27c8
> | x8 : 00000000ffffefff x7 : ffffaba8c4a0a7c8 x6 : 80000000fffff000
> | x5 : 0000000000000107 x4 : 0000000000000000 x3 : 0000000000000000
> | x2 : 0000000000000000 x1 : 0000000000000000 x0 : ffff3947c0eab3c0
> | Call trace:
> | buginit+0x18/0x1000 [hello]
> | do_one_initcall+0x80/0x1c8
> | do_init_module+0x60/0x218
> | load_module+0x1ba4/0x1d70
> | __do_sys_init_module+0x198/0x1d0
> | __arm64_sys_init_module+0x1c/0x28
> | invoke_syscall+0x48/0x114
> | el0_svc_common.constprop.0+0x40/0xe0
> | do_el0_svc+0x1c/0x28
> | el0_svc+0x34/0xd8
> | el0t_64_sync_handler+0x120/0x12c
> | el0t_64_sync+0x190/0x194
> | Code: d0ffffe0 910003fd 91000000 9400000b (d4210000)
> | ---[ end trace 0000000000000000 ]---
> | Kernel panic - not syncing: BRK handler: Fatal exception
> |
> | Fix this by always aligning the end of a bug_entry to 4 bytes, which is
> | correct regardless of CONFIG_DEBUG_BUGVERBOSE.
> |
> | Fixes: 9fb7410f955f ("arm64/BUG: Use BRK instruction for generic BUG traps")
> | Signed-off-by: Yuanbin Xie <[email protected]>
> | Signed-off-by: Jiangfeng Xiao <[email protected]>
>
> With that:
>
> Reviewed-by: Mark Rutland <[email protected]>
>
> Mark.
>
>>
>>
>>
>> diff --git a/arch/arm64/include/asm/asm-bug.h b/arch/arm64/include/asm/asm-bug.h
>> index c762038..6e73809 100644
>> --- a/arch/arm64/include/asm/asm-bug.h
>> +++ b/arch/arm64/include/asm/asm-bug.h
>> @@ -28,6 +28,7 @@
>> 14470: .long 14471f - .; \
>> _BUGVERBOSE_LOCATION(__FILE__, __LINE__) \
>> .short flags; \
>> + .align 2; \
The use of .align 2 here is based on the assumption that struct
bug_entry is 4-byte aligned. Currently, there is no problem with this
assumption, but for compatibility reasons, refer to the riscv
architecture and refactor the implementation of __BUG_FLAGS:
#define __BUG_FLAGS(flags) \
do { \
__asm__ __volatile__ ( \
"1:\n\t" \
"ebreak\n" \
".pushsection __bug_table,\"aw\"\n\t" \
"2:\n\t" \
__BUG_ENTRY "\n\t" \
".org 2b + %3\n\t" \
".popsection" \
: \
: "i" (__FILE__), "i" (__LINE__), \
"i" (flags), \
"i" (sizeof(struct bug_entry))); \
} while (0)
Align the real size of struct bug_entry through .org. What do you think?
>> .popsection; \
>> 14471:
>> #else
>> --
>> 1.8.5.6
>>
>>
>
On Mon, May 20, 2024 at 08:05:25PM +0800, Li Zetao wrote:
> On 2024/5/20 18:33, Mark Rutland wrote:
> > On Fri, May 17, 2024 at 10:13:28PM +0800, Jiangfeng Xiao wrote:
> > > diff --git a/arch/arm64/include/asm/asm-bug.h b/arch/arm64/include/asm/asm-bug.h
> > > index c762038..6e73809 100644
> > > --- a/arch/arm64/include/asm/asm-bug.h
> > > +++ b/arch/arm64/include/asm/asm-bug.h
> > > @@ -28,6 +28,7 @@
> > > 14470: .long 14471f - .; \
> > > _BUGVERBOSE_LOCATION(__FILE__, __LINE__) \
> > > .short flags; \
> > > + .align 2; \
> The use of .align 2 here is based on the assumption that struct bug_entry is
> 4-byte aligned. Currently, there is no problem with this assumption, but for
> compatibility reasons, refer to the riscv architecture and refactor the
> implementation of __BUG_FLAGS:
>
> #define __BUG_FLAGS(flags) \
> do { \
> __asm__ __volatile__ ( \
> "1:\n\t" \
> "ebreak\n" \
> ".pushsection __bug_table,\"aw\"\n\t" \
> "2:\n\t" \
> __BUG_ENTRY "\n\t" \
> ".org 2b + %3\n\t" \
> ".popsection" \
> : \
> : "i" (__FILE__), "i" (__LINE__), \
> "i" (flags), \
> "i" (sizeof(struct bug_entry))); \
> } while (0)
>
> Align the real size of struct bug_entry through .org. What do you think?
I can see why that appears nice, but we can't do that in asm-bug.h
without something in asm-offets.h, and I suspect that's going to lead to
pain with circular header dependencies.
Regardless, if things change we'll need to make other changes here, so I
don't think we gain much.
I think it's preferable to have the explicit .align for now.
Mark.
On 2024/5/20 20:05, Li Zetao wrote:
>>> diff --git a/arch/arm64/include/asm/asm-bug.h b/arch/arm64/include/asm/asm-bug.h
>>> index c762038..6e73809 100644
>>> --- a/arch/arm64/include/asm/asm-bug.h
>>> +++ b/arch/arm64/include/asm/asm-bug.h
>>> @@ -28,6 +28,7 @@
>>> 14470: .long 14471f - .; \
>>> _BUGVERBOSE_LOCATION(__FILE__, __LINE__) \
>>> .short flags; \
>>> + .align 2; \
> The use of .align 2 here is based on the assumption that struct bug_entry is 4-byte aligned. Currently, there is no problem with this assumption, but for compatibility reasons, refer to the riscv architecture and refactor the implementation of __BUG_FLAGS:
>
> #define __BUG_FLAGS(flags) \
> do { \
> __asm__ __volatile__ ( \
> "1:\n\t" \
> "ebreak\n" \
> ".pushsection __bug_table,\"aw\"\n\t" \
> "2:\n\t" \
> __BUG_ENTRY "\n\t" \
> ".org 2b + %3\n\t" \
> ".popsection" \
> : \
> : "i" (__FILE__), "i" (__LINE__), \
> "i" (flags), \
> "i" (sizeof(struct bug_entry))); \
> } while (0)
>
> Align the real size of struct bug_entry through .org. What do you think?
The implementation of risc-v BUG_ENTRY does handle
the `implicit padding` at the end of the struct correctly,
however, it does not handle the `implicit padding` in the middle of
the struct correctly, for example, assume that
the struct bug_entry changes as follows in the future:
struct bug_entry {
signed int bug_addr_disp; // 4 bytes
unsigned short flags; // 2 bytes
< implicit padding > // 6 bytes
unsigned long long flags2; // 8 bytes
}
Even the implementation of risc-v BUG_ENTRY
can't handle this situation. Referencing risc-v solution
complicates things, but doesn't completely solve the problem.
In the current scenario, we know the contents of struct bug_entry
and generate variables using assembly language.
I don't think it's necessary to complicate things.
On 2024/5/20 18:33, Mark Rutland wrote:
> I reckon we should spell out the reason for this a bit more; would you
> be happy with the commit message below?
>
> | When CONFIG_DEBUG_BUGVERBOSE=n, we fail to add necessary padding bytes
> | to bug_table entries, and as a result the last entry in a bug table will
> | be ignored, potentially leading to an unexpected panic(). All prior
> | entries in the table will be handled correctly.
> |
> | The arm64 ABI requires that struct fields of up to 8 bytes are
> | naturally-aligned, with padding added within a struct such that struct
> | are suitably aligned within arrays.
> |
> | When CONFIG_DEBUG_BUGVERPOSE=y, the layout of a bug_entry is:
> |
> | struct bug_entry {
> | signed int bug_addr_disp; // 4 bytes
> | signed int file_disp; // 4 bytes
> | unsigned short line; // 2 bytes
> | unsigned short flags; // 2 bytes
> | }
> |
> | ... with 12 bytes total, requiring 4-byte alignment.
> |
> | When CONFIG_DEBUG_BUGVERBOSE=n, the layout of a bug_entry is:
> |
> | struct bug_entry {
> | signed int bug_addr_disp; // 4 bytes
> | unsigned short flags; // 2 bytes
> | < implicit padding > // 2 bytes
> | }
> |
> | ... with 8 bytes total, with 6 bytes of data and 2 bytes of trailing
> | padding, requiring 4-byte alginment.
> |
> | When we create a bug_entry in assembly, we align the start of the entry
> | to 4 bytes, which implicitly handles padding for any prior entries.
> | However, we do not align the end of the entry, and so when
> | CONFIG_DEBUG_BUGVERBOSE=n, the final entry lacks the trailing padding
> | bytes.
> |
> | For the main kernel image this is not a problem as find_bug() doesn't
> | depend on the trailing padding bytes when searching for entries:
> |
> | for (bug = __start___bug_table; bug < __stop___bug_table; ++bug)
> | if (bugaddr == bug_addr(bug))
> | return bug;
> |
> | However for modules, module_bug_finalize() depends on the trailing
> | bytes when calculating the number of entries:
> |
> | mod->num_bugs = sechdrs[i].sh_size / sizeof(struct bug_entry);
> |
> | ... and as the last bug_entry lacks the necessary padding bytes, this entry
> | will not be counted, e.g. in the case of a single entry:
> |
> | sechdrs[i].sh_size == 6
> | sizeof(struct bug_entry) == 8;
> |
> | sechdrs[i].sh_size / sizeof(struct bug_entry) == 0;
> |
> | Consequently module_find_bug() will miss the last bug_entry when it does:
> |
> | for (i = 0; i < mod->num_bugs; ++i, ++bug)
> | if (bugaddr == bug_addr(bug))
> | goto out;
> |
> | ... which can lead to a kenrel panic due to an unhandled bug.
> |
> | This can be demonstrated with the following module:
> |
> | static int __init buginit(void)
> | {
> | WARN(1, "hello\n");
> | return 0;
> | }
> |
> | static void __exit bugexit(void)
> | {
> | }
> |
> | module_init(buginit);
> | module_exit(bugexit);
> | MODULE_LICENSE("GPL");
> |
> | ... which will trigger a kernel panic when loaded:
> |
> | ------------[ cut here ]------------
> | hello
> | Unexpected kernel BRK exception at EL1
> | Internal error: BRK handler: 00000000f2000800 [#1] PREEMPT SMP
> | Modules linked in: hello(O+)
> | CPU: 0 PID: 50 Comm: insmod Tainted: G O 6.9.1 #8
> | Hardware name: linux,dummy-virt (DT)
> | pstate: 60400005 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
> | pc : buginit+0x18/0x1000 [hello]
> | lr : buginit+0x18/0x1000 [hello]
> | sp : ffff800080533ae0
> | x29: ffff800080533ae0 x28: 0000000000000000 x27: 0000000000000000
> | x26: ffffaba8c4e70510 x25: ffff800080533c30 x24: ffffaba8c4a28a58
> | x23: 0000000000000000 x22: 0000000000000000 x21: ffff3947c0eab3c0
> | x20: ffffaba8c4e3f000 x19: ffffaba846464000 x18: 0000000000000006
> | x17: 0000000000000000 x16: ffffaba8c2492834 x15: 0720072007200720
> | x14: 0720072007200720 x13: ffffaba8c49b27c8 x12: 0000000000000312
> | x11: 0000000000000106 x10: ffffaba8c4a0a7c8 x9 : ffffaba8c49b27c8
> | x8 : 00000000ffffefff x7 : ffffaba8c4a0a7c8 x6 : 80000000fffff000
> | x5 : 0000000000000107 x4 : 0000000000000000 x3 : 0000000000000000
> | x2 : 0000000000000000 x1 : 0000000000000000 x0 : ffff3947c0eab3c0
> | Call trace:
> | buginit+0x18/0x1000 [hello]
> | do_one_initcall+0x80/0x1c8
> | do_init_module+0x60/0x218
> | load_module+0x1ba4/0x1d70
> | __do_sys_init_module+0x198/0x1d0
> | __arm64_sys_init_module+0x1c/0x28
> | invoke_syscall+0x48/0x114
> | el0_svc_common.constprop.0+0x40/0xe0
> | do_el0_svc+0x1c/0x28
> | el0_svc+0x34/0xd8
> | el0t_64_sync_handler+0x120/0x12c
> | el0t_64_sync+0x190/0x194
> | Code: d0ffffe0 910003fd 91000000 9400000b (d4210000)
> | ---[ end trace 0000000000000000 ]---
> | Kernel panic - not syncing: BRK handler: Fatal exception
> |
> | Fix this by always aligning the end of a bug_entry to 4 bytes, which is
> | correct regardless of CONFIG_DEBUG_BUGVERBOSE.
> |
> | Fixes: 9fb7410f955f ("arm64/BUG: Use BRK instruction for generic BUG traps")
> | Signed-off-by: Yuanbin Xie <[email protected]>
> | Signed-off-by: Jiangfeng Xiao <[email protected]>
>
> With that:
>
> Reviewed-by: Mark Rutland <[email protected]>
>
> Mark.
>
Thank you very much for your patience and professional review.
Indeed, we have not clearly expressed the specific layout
of `struct bug_entry` under the arm64 ABI
and how it causes the problem. We have only briefly described
that the assembly code does not consistent with
the binary layout of the C struct.
Your commit messge is much more accurate
in describing the problem, and we couldn't agree more.
I will follow your advice to submit our V2 patch.
When CONFIG_DEBUG_BUGVERBOSE=n, we fail to add necessary padding bytes
to bug_table entries, and as a result the last entry in a bug table will
be ignored, potentially leading to an unexpected panic(). All prior
entries in the table will be handled correctly.
The arm64 ABI requires that struct fields of up to 8 bytes are
naturally-aligned, with padding added within a struct such that struct
are suitably aligned within arrays.
When CONFIG_DEBUG_BUGVERPOSE=y, the layout of a bug_entry is:
struct bug_entry {
signed int bug_addr_disp; // 4 bytes
signed int file_disp; // 4 bytes
unsigned short line; // 2 bytes
unsigned short flags; // 2 bytes
}
.. with 12 bytes total, requiring 4-byte alignment.
When CONFIG_DEBUG_BUGVERBOSE=n, the layout of a bug_entry is:
struct bug_entry {
signed int bug_addr_disp; // 4 bytes
unsigned short flags; // 2 bytes
< implicit padding > // 2 bytes
}
.. with 8 bytes total, with 6 bytes of data and 2 bytes of trailing
padding, requiring 4-byte alginment.
When we create a bug_entry in assembly, we align the start of the entry
to 4 bytes, which implicitly handles padding for any prior entries.
However, we do not align the end of the entry, and so when
CONFIG_DEBUG_BUGVERBOSE=n, the final entry lacks the trailing padding
bytes.
For the main kernel image this is not a problem as find_bug() doesn't
depend on the trailing padding bytes when searching for entries:
for (bug = __start___bug_table; bug < __stop___bug_table; ++bug)
if (bugaddr == bug_addr(bug))
return bug;
However for modules, module_bug_finalize() depends on the trailing
bytes when calculating the number of entries:
mod->num_bugs = sechdrs[i].sh_size / sizeof(struct bug_entry);
.. and as the last bug_entry lacks the necessary padding bytes, this entry
will not be counted, e.g. in the case of a single entry:
sechdrs[i].sh_size == 6
sizeof(struct bug_entry) == 8;
sechdrs[i].sh_size / sizeof(struct bug_entry) == 0;
Consequently module_find_bug() will miss the last bug_entry when it does:
for (i = 0; i < mod->num_bugs; ++i, ++bug)
if (bugaddr == bug_addr(bug))
goto out;
.. which can lead to a kenrel panic due to an unhandled bug.
This can be demonstrated with the following module:
static int __init buginit(void)
{
WARN(1, "hello\n");
return 0;
}
static void __exit bugexit(void)
{
}
module_init(buginit);
module_exit(bugexit);
MODULE_LICENSE("GPL");
.. which will trigger a kernel panic when loaded:
------------[ cut here ]------------
hello
Unexpected kernel BRK exception at EL1
Internal error: BRK handler: 00000000f2000800 [#1] PREEMPT SMP
Modules linked in: hello(O+)
CPU: 0 PID: 50 Comm: insmod Tainted: G O 6.9.1 #8
Hardware name: linux,dummy-virt (DT)
pstate: 60400005 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
pc : buginit+0x18/0x1000 [hello]
lr : buginit+0x18/0x1000 [hello]
sp : ffff800080533ae0
x29: ffff800080533ae0 x28: 0000000000000000 x27: 0000000000000000
x26: ffffaba8c4e70510 x25: ffff800080533c30 x24: ffffaba8c4a28a58
x23: 0000000000000000 x22: 0000000000000000 x21: ffff3947c0eab3c0
x20: ffffaba8c4e3f000 x19: ffffaba846464000 x18: 0000000000000006
x17: 0000000000000000 x16: ffffaba8c2492834 x15: 0720072007200720
x14: 0720072007200720 x13: ffffaba8c49b27c8 x12: 0000000000000312
x11: 0000000000000106 x10: ffffaba8c4a0a7c8 x9 : ffffaba8c49b27c8
x8 : 00000000ffffefff x7 : ffffaba8c4a0a7c8 x6 : 80000000fffff000
x5 : 0000000000000107 x4 : 0000000000000000 x3 : 0000000000000000
x2 : 0000000000000000 x1 : 0000000000000000 x0 : ffff3947c0eab3c0
Call trace:
buginit+0x18/0x1000 [hello]
do_one_initcall+0x80/0x1c8
do_init_module+0x60/0x218
load_module+0x1ba4/0x1d70
__do_sys_init_module+0x198/0x1d0
__arm64_sys_init_module+0x1c/0x28
invoke_syscall+0x48/0x114
el0_svc_common.constprop.0+0x40/0xe0
do_el0_svc+0x1c/0x28
el0_svc+0x34/0xd8
el0t_64_sync_handler+0x120/0x12c
el0t_64_sync+0x190/0x194
Code: d0ffffe0 910003fd 91000000 9400000b (d4210000)
---[ end trace 0000000000000000 ]---
Kernel panic - not syncing: BRK handler: Fatal exception
Fix this by always aligning the end of a bug_entry to 4 bytes, which is
correct regardless of CONFIG_DEBUG_BUGVERBOSE.
Fixes: 9fb7410f955f ("arm64/BUG: Use BRK instruction for generic BUG traps")
Signed-off-by: Yuanbin Xie <[email protected]>
Signed-off-by: Jiangfeng Xiao <[email protected]>
Reviewed-by: Mark Rutland <[email protected]>
---
V1 -> V2: Describe the problem scenario and cause more clearly
in the commit message as suggested by the maintainer.
---
arch/arm64/include/asm/asm-bug.h | 1 +
1 file changed, 1 insertion(+)
diff --git a/arch/arm64/include/asm/asm-bug.h b/arch/arm64/include/asm/asm-bug.h
index c762038..6e73809 100644
--- a/arch/arm64/include/asm/asm-bug.h
+++ b/arch/arm64/include/asm/asm-bug.h
@@ -28,6 +28,7 @@
14470: .long 14471f - .; \
_BUGVERBOSE_LOCATION(__FILE__, __LINE__) \
.short flags; \
+ .align 2; \
.popsection; \
14471:
#else
--
1.8.5.6
On Mon, 20 May 2024 21:34:37 +0800, Jiangfeng Xiao wrote:
> When CONFIG_DEBUG_BUGVERBOSE=n, we fail to add necessary padding bytes
> to bug_table entries, and as a result the last entry in a bug table will
> be ignored, potentially leading to an unexpected panic(). All prior
> entries in the table will be handled correctly.
>
> The arm64 ABI requires that struct fields of up to 8 bytes are
> naturally-aligned, with padding added within a struct such that struct
> are suitably aligned within arrays.
>
> [...]
Applied to arm64 (for-next/core), thanks!
[1/1] arm64: asm-bug: Add .align 2 to the end of __BUG_ENTRY
https://git.kernel.org/arm64/c/ffbf4fb9b5c1
Cheers,
--
Will
https://fixes.arm64.dev
https://next.arm64.dev
https://will.arm64.dev