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Message-ID: <20d70835-9411-9a08-c567-56d7040e01dd@huawei.com>
Date: Mon, 20 May 2024 20:05:25 +0800
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Subject: Re: [PATCH] arm64: asm-bug: Add .align 2 to the end of __BUG_ENTRY
To: Mark Rutland <mark.rutland@arm.com>, Jiangfeng Xiao
	<xiaojiangfeng@huawei.com>
CC: <catalin.marinas@arm.com>, <will@kernel.org>, <Dave.Martin@arm.com>,
	<xieyuanbin1@huawei.com>, <linux-arm-kernel@lists.infradead.org>,
	<linux-kernel@vger.kernel.org>, <nixiaoming@huawei.com>,
	<wangbing6@huawei.com>, <douzhaolei@huawei.com>, <liaohua4@huawei.com>,
	<lijiahuan5@huawei.com>, <wangfangpeng1@huawei.com>, "zhangjianhua (E)"
	<chris.zjh@huawei.com>
References: <1715955208-17109-1-git-send-email-xiaojiangfeng@huawei.com>
 <ZksnARrLkKHcX4C_@J2N7QTR9R3>
Content-Language: en-US
From: Li Zetao <lizetao1@huawei.com>
In-Reply-To: <ZksnARrLkKHcX4C_@J2N7QTR9R3>
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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 <xieyuanbin1@huawei.com>
>> Signed-off-by: Jiangfeng Xiao <xiaojiangfeng@huawei.com>
>> ---
>>   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 <xieyuanbin1@huawei.com>
> | Signed-off-by: Jiangfeng Xiao <xiaojiangfeng@huawei.com>
> 
> With that:
> 
> Reviewed-by: Mark Rutland <mark.rutland@arm.com>
> 
> 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
>>
>>
>