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

[Will Deacon <will@kernel.org>]

On Fri, Nov 28, 2025 at 09:39:45AM +0800, Zizhi Wo wrote:
> 在 2025/11/28 9:18, Zizhi Wo 写道:
> > 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?
> 
> I tried it out and it works — thank you for the solution you provided.

Thanks for giving it a spin.

> At the same time, since I’m a beginner in this area, I’d like to ask a
> question.
> 
> The comment above do_translation_fault() says:
> “We enter here because the first level page table doesn't contain a
> valid entry for the address.”
> 
> However, after modifying the code, it seems that when encountering
> FSR_FS_INVALID_PAGE, the kernel no longer creates a page table entry,
> but instead directly jumps to bad_area.

FSR_FS_INVALID_PAGE indicates a last level translation fault (that's the
"page" part) so it's only applicable in the case where the other levels
of page-table have been populated already.

I wondered about checking !is_vmalloc_addr() too, but I couldn't
convince myself that load_unaligned_zeropad() is only ever used with the
linear map.

> I'd like to ask — could this change potentially cause any other side
> effects?

There's always the possibility but I personally think it's more
self-contained than the other patches doing the rounds. For example, I
don't make any changes to the permission fault handling path.

Will