Re: [PATCH v2 mm-stable] procfs: avoid fetching build ID while holding VMA lock

[Shakeel Butt <shakeel.butt@linux.dev>]

On Thu, Jan 29, 2026 at 1:53 PM Andrii Nakryiko <andrii@kernel.org> wrote:
>
> Fix PROCMAP_QUERY to fetch optional build ID only after dropping mmap_lock or
> per-VMA lock, whichever was used to lock VMA under question, to avoid deadlock
> reported by syzbot:
>
>  -> #1 (&mm->mmap_lock){++++}-{4:4}:
>         __might_fault+0xed/0x170
>         _copy_to_iter+0x118/0x1720
>         copy_page_to_iter+0x12d/0x1e0
>         filemap_read+0x720/0x10a0
>         blkdev_read_iter+0x2b5/0x4e0
>         vfs_read+0x7f4/0xae0
>         ksys_read+0x12a/0x250
>         do_syscall_64+0xcb/0xf80
>         entry_SYSCALL_64_after_hwframe+0x77/0x7f
>
>  -> #0 (&sb->s_type->i_mutex_key#8){++++}-{4:4}:
>         __lock_acquire+0x1509/0x26d0
>         lock_acquire+0x185/0x340
>         down_read+0x98/0x490
>         blkdev_read_iter+0x2a7/0x4e0
>         __kernel_read+0x39a/0xa90
>         freader_fetch+0x1d5/0xa80
>         __build_id_parse.isra.0+0xea/0x6a0
>         do_procmap_query+0xd75/0x1050
>         procfs_procmap_ioctl+0x7a/0xb0
>         __x64_sys_ioctl+0x18e/0x210
>         do_syscall_64+0xcb/0xf80
>         entry_SYSCALL_64_after_hwframe+0x77/0x7f
>
>  other info that might help us debug this:
>
>   Possible unsafe locking scenario:
>
>         CPU0                    CPU1
>         ----                    ----
>    rlock(&mm->mmap_lock);
>                                 lock(&sb->s_type->i_mutex_key#8);
>                                 lock(&mm->mmap_lock);
>    rlock(&sb->s_type->i_mutex_key#8);
>
>   *** DEADLOCK ***
>
> To make this safe, we need to grab file refcount while VMA is still locked, but
> other than that everything is pretty straightforward. Internal build_id_parse()
> API assumes VMA is passed, but it only needs the underlying file reference, so
> just add another variant build_id_parse_file() that expects file passed
> directly.
>
> Fixes: ed5d583a88a9 ("fs/procfs: implement efficient VMA querying API for /proc/<pid>/maps")
> Reported-by: syzbot+4e70c8e0a2017b432f7a@syzkaller.appspotmail.com
> Reviewed-by: Suren Baghdasaryan <surenb@google.com>
> Tested-by: Suren Baghdasaryan <surenb@google.com>
> Signed-off-by: Andrii Nakryiko <andrii@kernel.org>

Reviewed-by: Shakeel Butt <shakeel.butt@linux.dev>