From: Hou Tao <[email protected]>
When trying to use copy_from_kernel_nofault() to read vsyscall page
through a bpf program, the following oops was reported:
BUG: unable to handle page fault for address: ffffffffff600000
#PF: supervisor read access in kernel mode
#PF: error_code(0x0000) - not-present page
PGD 3231067 P4D 3231067 PUD 3233067 PMD 3235067 PTE 0
Oops: 0000 [#1] PREEMPT SMP PTI
CPU: 1 PID: 20390 Comm: test_progs ...... 6.7.0+ #58
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996) ......
RIP: 0010:copy_from_kernel_nofault+0x6f/0x110
......
Call Trace:
<TASK>
? copy_from_kernel_nofault+0x6f/0x110
bpf_probe_read_kernel+0x1d/0x50
bpf_prog_2061065e56845f08_do_probe_read+0x51/0x8d
trace_call_bpf+0xc5/0x1c0
perf_call_bpf_enter.isra.0+0x69/0xb0
perf_syscall_enter+0x13e/0x200
syscall_trace_enter+0x188/0x1c0
do_syscall_64+0xb5/0xe0
entry_SYSCALL_64_after_hwframe+0x6e/0x76
</TASK>
......
---[ end trace 0000000000000000 ]---
It seems the occurrence of oops depends on SMAP feature of CPU. It
happens as follow: a bpf program uses bpf_probe_read_kernel() to read
from vsyscall page, bpf_probe_read_kernel() invokes
copy_from_kernel_nofault() in turn and then invokes __get_user_asm().
Because the vsyscall page address is not readable for kernel space,
a page fault exception is triggered accordingly, handle_page_fault()
considers the vsyscall page address as a userspace address instead of a
kernel space address, so the fix-up set-up by bpf isn't applied. Because
the CPU has SMAP feature and the access happens in kernel mode, so
page_fault_oops() is invoked and an oops happens. If these is no SMAP
feature, the fix-up set-up by bpf will be applied and
copy_from_kernel_nofault() will return -EFAULT instead.
Considering handle_page_fault() has already considered the vsyscall page
address as a userspace address, fix the problem by disallowing vsyscall
page read for copy_from_kernel_nofault().
Originally-by: Thomas Gleixner <[email protected]>
Reported-by: [email protected]
Closes: https://lore.kernel.org/bpf/CAG48ez06TZft=ATH1qh2c5mpS5BT8UakwNkzi6nvK5_djC-4Nw@mail.gmail.com
Reported-by: xingwei lee <[email protected]>
Closes: https://lore.kernel.org/bpf/CABOYnLynjBoFZOf3Z4BhaZkc5hx_kHfsjiW+UWLoB=w33LvScw@mail.gmail.com
Signed-off-by: Hou Tao <[email protected]>
---
arch/x86/mm/maccess.c | 9 +++++++++
1 file changed, 9 insertions(+)
diff --git a/arch/x86/mm/maccess.c b/arch/x86/mm/maccess.c
index 6993f026adec9..d9272e1db5224 100644
--- a/arch/x86/mm/maccess.c
+++ b/arch/x86/mm/maccess.c
@@ -3,6 +3,8 @@
#include <linux/uaccess.h>
#include <linux/kernel.h>
+#include <asm/vsyscall.h>
+
#ifdef CONFIG_X86_64
bool copy_from_kernel_nofault_allowed(const void *unsafe_src, size_t size)
{
@@ -15,6 +17,13 @@ bool copy_from_kernel_nofault_allowed(const void *unsafe_src, size_t size)
if (vaddr < TASK_SIZE_MAX + PAGE_SIZE)
return false;
+ /* Also consider the vsyscall page as userspace address. Otherwise,
+ * reading the vsyscall page in copy_from_kernel_nofault() may
+ * trigger an oops due to an unhandled page fault.
+ */
+ if (is_vsyscall_vaddr(vaddr))
+ return false;
+
/*
* Allow everything during early boot before 'x86_virt_bits'
* is initialized. Needed for instruction decoding in early
--
2.29.2
Hi Hou Tao,
I agree to your approach in this patch. Please see some comments below.
On 1/26/2024 3:54 AM, Hou Tao wrote:
> From: Hou Tao <[email protected]>
>
> When trying to use copy_from_kernel_nofault() to read vsyscall page
> through a bpf program, the following oops was reported:
>
> BUG: unable to handle page fault for address: ffffffffff600000
> #PF: supervisor read access in kernel mode
> #PF: error_code(0x0000) - not-present page
> PGD 3231067 P4D 3231067 PUD 3233067 PMD 3235067 PTE 0
> Oops: 0000 [#1] PREEMPT SMP PTI
> CPU: 1 PID: 20390 Comm: test_progs ...... 6.7.0+ #58
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996) ......
> RIP: 0010:copy_from_kernel_nofault+0x6f/0x110
> ......
> Call Trace:
> <TASK>
> ? copy_from_kernel_nofault+0x6f/0x110
> bpf_probe_read_kernel+0x1d/0x50
> bpf_prog_2061065e56845f08_do_probe_read+0x51/0x8d
> trace_call_bpf+0xc5/0x1c0
> perf_call_bpf_enter.isra.0+0x69/0xb0
> perf_syscall_enter+0x13e/0x200
> syscall_trace_enter+0x188/0x1c0
> do_syscall_64+0xb5/0xe0
> entry_SYSCALL_64_after_hwframe+0x6e/0x76
> </TASK>
> ......
> ---[ end trace 0000000000000000 ]---
>
> It seems the occurrence of oops depends on SMAP feature of CPU. It
> happens as follow: a bpf program uses bpf_probe_read_kernel() to read
> from vsyscall page, bpf_probe_read_kernel() invokes
> copy_from_kernel_nofault() in turn and then invokes __get_user_asm().
> Because the vsyscall page address is not readable for kernel space,
> a page fault exception is triggered accordingly, handle_page_fault()
> considers the vsyscall page address as a userspace address instead of a
> kernel space address, so the fix-up set-up by bpf isn't applied. Because
> the CPU has SMAP feature and the access happens in kernel mode, so
> page_fault_oops() is invoked and an oops happens. If these is no SMAP
> feature, the fix-up set-up by bpf will be applied and
> copy_from_kernel_nofault() will return -EFAULT instead.
>
I find this paragraph to be a bit hard to follow. I think we can
minimize the reference to SMAP here since it is only helping detect
cross address space accesses. How about something like the following:
The oops is triggered when:
1) A bpf program uses bpf_probe_read_kernel() to read from the vsyscall
page and invokes copy_from_kernel_nofault() which in turn calls
__get_user_asm().
2) Because the vsyscall page address is not readable from kernel space,
a page fault exception is triggered accordingly.
3) handle_page_fault() considers the vsyscall page address as a user
space address instead of a kernel space address. This results in the
fix-up setup by bpf not being applied and a page_fault_oops() is invoked
due to SMAP.
> Considering handle_page_fault() has already considered the vsyscall page
> address as a userspace address, fix the problem by disallowing vsyscall
> page read for copy_from_kernel_nofault().
>
I agree, following the same approach as handle_page_fault() seems
reasonable.
> Originally-by: Thomas Gleixner <[email protected]>
> Reported-by: [email protected]
> Closes: https://lore.kernel.org/bpf/CAG48ez06TZft=ATH1qh2c5mpS5BT8UakwNkzi6nvK5_djC-4Nw@mail.gmail.com
> Reported-by: xingwei lee <[email protected]>
> Closes: https://lore.kernel.org/bpf/CABOYnLynjBoFZOf3Z4BhaZkc5hx_kHfsjiW+UWLoB=w33LvScw@mail.gmail.com
> Signed-off-by: Hou Tao <[email protected]>
> ---
> arch/x86/mm/maccess.c | 9 +++++++++
> 1 file changed, 9 insertions(+)
>
> diff --git a/arch/x86/mm/maccess.c b/arch/x86/mm/maccess.c
> index 6993f026adec9..d9272e1db5224 100644
> --- a/arch/x86/mm/maccess.c
> +++ b/arch/x86/mm/maccess.c
> @@ -3,6 +3,8 @@
> #include <linux/uaccess.h>
> #include <linux/kernel.h>
>
> +#include <asm/vsyscall.h>
> +
> #ifdef CONFIG_X86_64
> bool copy_from_kernel_nofault_allowed(const void *unsafe_src, size_t size)
> {
> @@ -15,6 +17,13 @@ bool copy_from_kernel_nofault_allowed(const void *unsafe_src, size_t size)
> if (vaddr < TASK_SIZE_MAX + PAGE_SIZE)
> return false;
>
> + /* Also consider the vsyscall page as userspace address. Otherwise,
> + * reading the vsyscall page in copy_from_kernel_nofault() may
> + * trigger an oops due to an unhandled page fault.
> + */
x86 prefers a slightly different style for multi-line comments. Please
refer to https://docs.kernel.org/process/maintainer-tip.html#comment-style.
How about rewording the above as:
/*
* Reading from the vsyscall page may cause an unhandled fault in
* certain cases. Though it is at an address above TASK_SIZE_MAX, it is
* usually considered as a user space address.
*/
> + if (is_vsyscall_vaddr(vaddr))
> + return false;
> +
It would have been convenient if we had a common check for whether a
particular address is a kernel address or not. fault_in_kernel_space()
serves that purpose to an extent in other places.
I thought we could rename fault_in_kernel_space() to
vaddr_in_kernel_space() and use it here. But the check in
copy_from_kernel_nofault_allowed() includes the user guard page as well.
So the checks wouldn't exactly be the same.
I am unsure of the implications if we get rid of that difference. Maybe
we can leave it as-is for now unless someone else chimes in.
Sohil
Hi,
On 1/30/2024 7:50 AM, Sohil Mehta wrote:
> Hi Hou Tao,
>
> I agree to your approach in this patch. Please see some comments below.
>
> On 1/26/2024 3:54 AM, Hou Tao wrote:
>> From: Hou Tao <[email protected]>
>>
>> When trying to use copy_from_kernel_nofault() to read vsyscall page
>> through a bpf program, the following oops was reported:
[SNIP]
>> It seems the occurrence of oops depends on SMAP feature of CPU. It
>> happens as follow: a bpf program uses bpf_probe_read_kernel() to read
>> from vsyscall page, bpf_probe_read_kernel() invokes
>> copy_from_kernel_nofault() in turn and then invokes __get_user_asm().
>> Because the vsyscall page address is not readable for kernel space,
>> a page fault exception is triggered accordingly, handle_page_fault()
>> considers the vsyscall page address as a userspace address instead of a
>> kernel space address, so the fix-up set-up by bpf isn't applied. Because
>> the CPU has SMAP feature and the access happens in kernel mode, so
>> page_fault_oops() is invoked and an oops happens. If these is no SMAP
>> feature, the fix-up set-up by bpf will be applied and
>> copy_from_kernel_nofault() will return -EFAULT instead.
>>
> I find this paragraph to be a bit hard to follow. I think we can
> minimize the reference to SMAP here since it is only helping detect
> cross address space accesses. How about something like the following:
>
> The oops is triggered when:
>
> 1) A bpf program uses bpf_probe_read_kernel() to read from the vsyscall
> page and invokes copy_from_kernel_nofault() which in turn calls
> __get_user_asm().
>
> 2) Because the vsyscall page address is not readable from kernel space,
> a page fault exception is triggered accordingly.
>
> 3) handle_page_fault() considers the vsyscall page address as a user
> space address instead of a kernel space address. This results in the
> fix-up setup by bpf not being applied and a page_fault_oops() is invoked
> due to SMAP.
Thanks for the rephrasing. It is much better now.
>> Considering handle_page_fault() has already considered the vsyscall page
>> address as a userspace address, fix the problem by disallowing vsyscall
>> page read for copy_from_kernel_nofault().
>>
> I agree, following the same approach as handle_page_fault() seems
> reasonable.
>
>> Originally-by: Thomas Gleixner <[email protected]>
>> Reported-by: [email protected]
>> Closes: https://lore.kernel.org/bpf/CAG48ez06TZft=ATH1qh2c5mpS5BT8UakwNkzi6nvK5_djC-4Nw@mail.gmail.com
>> Reported-by: xingwei lee <[email protected]>
>> Closes: https://lore.kernel.org/bpf/CABOYnLynjBoFZOf3Z4BhaZkc5hx_kHfsjiW+UWLoB=w33LvScw@mail.gmail.com
>> Signed-off-by: Hou Tao <[email protected]>
>> ---
>> arch/x86/mm/maccess.c | 9 +++++++++
>> 1 file changed, 9 insertions(+)
>>
>> diff --git a/arch/x86/mm/maccess.c b/arch/x86/mm/maccess.c
>> index 6993f026adec9..d9272e1db5224 100644
>> --- a/arch/x86/mm/maccess.c
>> +++ b/arch/x86/mm/maccess.c
>> @@ -3,6 +3,8 @@
>> #include <linux/uaccess.h>
>> #include <linux/kernel.h>
>>
>> +#include <asm/vsyscall.h>
>> +
>> #ifdef CONFIG_X86_64
>> bool copy_from_kernel_nofault_allowed(const void *unsafe_src, size_t size)
>> {
>> @@ -15,6 +17,13 @@ bool copy_from_kernel_nofault_allowed(const void *unsafe_src, size_t size)
>> if (vaddr < TASK_SIZE_MAX + PAGE_SIZE)
>> return false;
>>
>> + /* Also consider the vsyscall page as userspace address. Otherwise,
>> + * reading the vsyscall page in copy_from_kernel_nofault() may
>> + * trigger an oops due to an unhandled page fault.
>> + */
> x86 prefers a slightly different style for multi-line comments. Please
> refer to https://docs.kernel.org/process/maintainer-tip.html#comment-style.
I see. Will update.
>
> How about rewording the above as:
>
> /*
> * Reading from the vsyscall page may cause an unhandled fault in
> * certain cases. Though it is at an address above TASK_SIZE_MAX, it is
> * usually considered as a user space address.
> */
Thanks for the rewording. Will do in v3.
>
>> + if (is_vsyscall_vaddr(vaddr))
>> + return false;
>> +
> It would have been convenient if we had a common check for whether a
> particular address is a kernel address or not. fault_in_kernel_space()
> serves that purpose to an extent in other places.
>
> I thought we could rename fault_in_kernel_space() to
> vaddr_in_kernel_space() and use it here. But the check in
> copy_from_kernel_nofault_allowed() includes the user guard page as well.
> So the checks wouldn't exactly be the same.
>
> I am unsure of the implications if we get rid of that difference. Maybe
> we can leave it as-is for now unless someone else chimes in.
There is other difference between fault_in_kernel_space() and
copy_from_kernel_nofault_allowed(). fault_in_kernel_space() uses address
>= TASK_SIZE_MAX to check the kernel space address, but
copy_from_kernel_nofault_allowed() uses vaddr >= TASK_SIZE_MAX +
PAGE_SIZE to check the kernel space address, so I prefer to keep it as-is.
>
> Sohil