linux-kernel - Re: [syzbot] [mm?] possible deadlock in __mmap_lock_do_trace

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Message-ID: <e0c7a2f8-9147-42bd-b531-f51c4d0c5082@I-love.SAKURA.ne.jp>
Date: Thu, 4 Jul 2024 22:21:50 +0900
From: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To: Axel Rasmussen <axelrasmussen@...gle.com>
Cc: syzbot+16b6ab88e66b34d09014@...kaller.appspotmail.com,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        linux-mm <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        nsaenz@...zon.com
Subject: Re: [syzbot] [mm?] possible deadlock in __mmap_lock_do_trace_released

On 2024/07/03 7:35, Axel Rasmussen wrote:
> But, note that stack_map_get_build_id calls mmap_read_trylock, so I
> would expect in the recursive case that call will simply fail, and
> then stack_map_get_build_id_offset appears to deal gracefully with
> that?

Unless that mmap was already held for write (or someone has started waiting
to hold it for write), recursive mmap_read_trylock() will succeed. Thus,
unless there is a guarantee of no infinite recursion, we should implement
a safeguard based on the worst scenario.