Re: [Bug report] hash_name() may cross page boundary and trigger sleep in RCU context

[Zizhi Wo <wozizhi@huaweicloud.com>]

在 2025/11/28 9:17, Zizhi Wo 写道:
> 
> 
> 在 2025/11/27 20:59, Will Deacon 写道:
>> On Wed, Nov 26, 2025 at 05:05:05PM +0800, Zizhi Wo wrote:
>>> We're running into the following issue on an ARM32 platform with the 
>>> linux
>>> 5.10 kernel:
>>>
>>> [<c0300b78>] (__dabt_svc) from [<c0529cb8>] 
>>> (link_path_walk.part.7+0x108/0x45c)
>>> [<c0529cb8>] (link_path_walk.part.7) from [<c052a948>] 
>>> (path_openat+0xc4/0x10ec)
>>> [<c052a948>] (path_openat) from [<c052cf90>] (do_filp_open+0x9c/0x114)
>>> [<c052cf90>] (do_filp_open) from [<c0511e4c>] 
>>> (do_sys_openat2+0x418/0x528)
>>> [<c0511e4c>] (do_sys_openat2) from [<c0513d98>] (do_sys_open+0x88/0xe4)
>>> [<c0513d98>] (do_sys_open) from [<c03000c0>] (ret_fast_syscall+0x0/0x58)
>>> ...
>>> [<c0315e34>] (unwind_backtrace) from [<c030f2b0>] (show_stack+0x20/0x24)
>>> [<c030f2b0>] (show_stack) from [<c14239f4>] (dump_stack+0xd8/0xf8)
>>> [<c14239f4>] (dump_stack) from [<c038d188>] (___might_sleep+0x19c/0x1e4)
>>> [<c038d188>] (___might_sleep) from [<c031b6fc>] 
>>> (do_page_fault+0x2f8/0x51c)
>>> [<c031b6fc>] (do_page_fault) from [<c031bb44>] (do_DataAbort+0x90/0x118)
>>> [<c031bb44>] (do_DataAbort) from [<c0300b78>] (__dabt_svc+0x58/0x80)
>>> ...
>>>
>>> During the execution of hash_name()->load_unaligned_zeropad(), a 
>>> potential
>>> memory access beyond the PAGE boundary may occur. For example, when the
>>> filename length is near the PAGE_SIZE boundary. This triggers a page 
>>> fault,
>>> which leads to a call to do_page_fault()->mmap_read_trylock(). If we 
>>> can't
>>> acquire the lock, we have to fall back to the mmap_read_lock() path, 
>>> which
>>> calls might_sleep(). This breaks RCU semantics because path lookup 
>>> occurs
>>> under an RCU read-side critical section. In linux-mainline, arm/arm64
>>> do_page_fault() still has this problem:
>>>
>>> lock_mm_and_find_vma->get_mmap_lock_carefully->mmap_read_lock_killable.
>>>
>>> And before commit bfcfaa77bdf0 ("vfs: use 'unsigned long' accesses for
>>> dcache name comparison and hashing"), hash_name accessed the name 
>>> byte by
>>> byte.
>>>
>>> To prevent load_unaligned_zeropad() from accessing beyond the valid 
>>> memory
>>> region, we would need to intercept such cases beforehand? But doing so
>>> would require replicating the internal logic of 
>>> load_unaligned_zeropad(),
>>> including handling endianness and constructing the correct value 
>>> manually.
>>> Given that load_unaligned_zeropad() is used in many places across the
>>> kernel, we currently haven't found a good solution to address this 
>>> cleanly.
>>>
>>> What would be the recommended way to handle this situation? Would
>>> appreciate any feedback and guidance from the community. Thanks!
>>
>> Does it help if you bodge the translation fault handler along the lines
>> of the untested diff below?
> 
> Thank you for the solution you provided. However, I seem to have
> encountered a bit of a problem.
> 
>>
>> Will
>>
>> --->8
>>
>> diff --git a/arch/arm/mm/fault.c b/arch/arm/mm/fault.c
>> index bf1577216ffa..b3c81e448798 100644
>> --- a/arch/arm/mm/fault.c
>> +++ b/arch/arm/mm/fault.c
>> @@ -407,7 +407,7 @@ do_translation_fault(unsigned long addr, unsigned 
>> int fsr,
>>          if (addr < TASK_SIZE)
>>                  return do_page_fault(addr, fsr, regs);
>> -       if (user_mode(regs))
>> +       if (user_mode(regs) || fsr_fs(fsr) == FSR_FS_INVALID_PAGE)
>>                  goto bad_area;
> 
> 
> 
> I'm getting an "FSR_FS_INVALID_PAGE undeclared" error during
> compilation...
> 
> In which kernel or FSR version was this macro or constant defined

Sorry, I didn't see this "#define FSR_FS_INVALID_PAGE". I'll try again
right away.

Please ignore my previous reply.

> 
>>          index = pgd_index(addr);
>> diff --git a/arch/arm/mm/fault.h b/arch/arm/mm/fault.h
>> index 9ecc2097a87a..8fb26f85e361 100644
>> --- a/arch/arm/mm/fault.h
>> +++ b/arch/arm/mm/fault.h
>> @@ -12,6 +12,8 @@
>>   #define FSR_FS3_0              (15)
>>   #define FSR_FS5_0              (0x3f)
>> +#define FSR_FS_INVALID_PAGE    7
>> +
>>   #ifdef CONFIG_ARM_LPAE
>>   #define FSR_FS_AEA             17
>> diff --git a/arch/arm/mm/fsr-2level.c b/arch/arm/mm/fsr-2level.c
>> index f2be95197265..c7060da345df 100644
>> --- a/arch/arm/mm/fsr-2level.c
>> +++ b/arch/arm/mm/fsr-2level.c
>> @@ -11,7 +11,7 @@ static struct fsr_info fsr_info[] = {
>>          { do_bad,               SIGBUS,  0,             "external 
>> abort on linefetch"      },
>>          { do_translation_fault, SIGSEGV, SEGV_MAPERR,   "section 
>> translation fault"        },
>>          { do_bad,               SIGBUS,  0,             "external 
>> abort on linefetch"      },
>> -       { do_page_fault,        SIGSEGV, SEGV_MAPERR,   "page 
>> translation fault"           },
>> +       { do_translation_fault, SIGSEGV, SEGV_MAPERR,   "page 
>> translation fault"           },
>>          { do_bad,               SIGBUS,  0,             "external 
>> abort on non-linefetch"  },
>>          { do_bad,               SIGSEGV, SEGV_ACCERR,   "section 
>> domain fault"             },
>>          { do_bad,               SIGBUS,  0,             "external 
>> abort on non-linefetch"  },
>> diff --git a/arch/arm/mm/fsr-3level.c b/arch/arm/mm/fsr-3level.c
>> index d0ae2963656a..19df4af828bd 100644
>> --- a/arch/arm/mm/fsr-3level.c
>> +++ b/arch/arm/mm/fsr-3level.c
>> @@ -7,7 +7,7 @@ static struct fsr_info fsr_info[] = {
>>          { do_bad,               SIGBUS,  0,             "reserved 
>> translation fault"    },
>>          { do_translation_fault, SIGSEGV, SEGV_MAPERR,   "level 1 
>> translation fault"     },
>>          { do_translation_fault, SIGSEGV, SEGV_MAPERR,   "level 2 
>> translation fault"     },
>> -       { do_page_fault,        SIGSEGV, SEGV_MAPERR,   "level 3 
>> translation fault"     },
>> +       { do_translation_fault, SIGSEGV, SEGV_MAPERR,   "level 3 
>> translation fault"     },
>>          { do_bad,               SIGBUS,  0,             "reserved 
>> access flag fault"    },
>>          { do_bad,               SIGSEGV, SEGV_ACCERR,   "level 1 
>> access flag fault"     },
>>          { do_page_fault,        SIGSEGV, SEGV_ACCERR,   "level 2 
>> access flag fault"     },
>>
>>
> 
> By the way, I tried Al's solution, and this problem didn't reproduce.
> 
> Thanks,
> Zizhi Wo