Re: [PATCH v2] binder: fix descriptor lookup for context manager

[Carlos Llamas <cmllamas@google.com>]

On Mon, Jul 22, 2024 at 03:05:11PM +0000, Carlos Llamas wrote:
> In commit 15d9da3f818c ("binder: use bitmap for faster descriptor
> lookup"), it was incorrectly assumed that references to the context
> manager node should always get descriptor zero assigned to them.
> 
> However, if the context manager dies and a new process takes its place,
> then assigning descriptor zero to the new context manager might lead to
> collisions, as there could still be references to the older node. This
> issue was reported by syzbot with the following trace:
> 
>   kernel BUG at drivers/android/binder.c:1173!
>   Internal error: Oops - BUG: 00000000f2000800 [#1] PREEMPT SMP
>   Modules linked in:
>   CPU: 1 PID: 447 Comm: binder-util Not tainted 6.10.0-rc6-00348-g31643d84b8c3 #10
>   Hardware name: linux,dummy-virt (DT)
>   pstate: 60000005 (nZCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
>   pc : binder_inc_ref_for_node+0x500/0x544
>   lr : binder_inc_ref_for_node+0x1e4/0x544
>   sp : ffff80008112b940
>   x29: ffff80008112b940 x28: ffff0e0e40310780 x27: 0000000000000000
>   x26: 0000000000000001 x25: ffff0e0e40310738 x24: ffff0e0e4089ba34
>   x23: ffff0e0e40310b00 x22: ffff80008112bb50 x21: ffffaf7b8f246970
>   x20: ffffaf7b8f773f08 x19: ffff0e0e4089b800 x18: 0000000000000000
>   x17: 0000000000000000 x16: 0000000000000000 x15: 000000002de4aa60
>   x14: 0000000000000000 x13: 2de4acf000000000 x12: 0000000000000020
>   x11: 0000000000000018 x10: 0000000000000020 x9 : ffffaf7b90601000
>   x8 : ffff0e0e48739140 x7 : 0000000000000000 x6 : 000000000000003f
>   x5 : ffff0e0e40310b28 x4 : 0000000000000000 x3 : ffff0e0e40310720
>   x2 : ffff0e0e40310728 x1 : 0000000000000000 x0 : ffff0e0e40310710
>   Call trace:
>    binder_inc_ref_for_node+0x500/0x544
>    binder_transaction+0xf68/0x2620
>    binder_thread_write+0x5bc/0x139c
>    binder_ioctl+0xef4/0x10c8
>   [...]
> 
> This patch adds back the previous behavior of assigning the next
> non-zero descriptor if references to previous context managers still
> exist. It amends both strategies, the newer dbitmap code and also the
> legacy slow_desc_lookup_olocked(), by allowing them to start looking
> for available descriptors at a given offset.
> 
> Fixes: 15d9da3f818c ("binder: use bitmap for faster descriptor lookup")
> Cc: stable@vger.kernel.org
> Reported-and-tested-by: syzbot+3dae065ca76952a67257@syzkaller.appspotmail.com
> Closes: https://lore.kernel.org/all/000000000000c1c0a0061d1e6979@google.com/
> Reviewed-by: Alice Ryhl <aliceryhl@google.com>
> Signed-off-by: Carlos Llamas <cmllamas@google.com>
> ---

Sorry, I forgot to feed --notes to git send-email and the list of
changes in this v2 patch was missed. Here it is:

Notes:
    v2: updated comment about BIT(0) per Alice's feedback
        collect tags