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

[Zizhi Wo <wozizhi@huaweicloud.com>]

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?

>   
>          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