Re: [PATCH] scsi: sg: fix slab-use-after-free Read in sg_release

Suraj Sonawane
2024-11-25T14:30:55Z

On 11/20/24 18:29, Suraj Sonawane wrote:
> Fix a use-after-free bug in `sg_release`,
> detected by syzbot with KASAN:
>
> BUG: KASAN: slab-use-after-free in lock_release+0x151/0xa30
> kernel/locking/lockdep.c:5838
> __mutex_unlock_slowpath+0xe2/0x750 kernel/locking/mutex.c:912
> sg_release+0x1f4/0x2e0 drivers/scsi/sg.c:407
>
> Root Cause:
> In `sg_release`, the function `kref_put(&sfp->f_ref, sg_remove_sfp)`
> is called before releasing the `open_rel_lock` mutex. The `kref_put`
> call may decrement the reference count of `sfp` to zero, triggering
> its cleanup through `sg_remove_sfp`. This cleanup includes scheduling
> deferred work via `sg_remove_sfp_usercontext`, which ultimately frees
> `sfp`.
>
> After `kref_put`, `sg_release` continues to unlock `open_rel_lock` and
> may reference `sfp` or `sdp`. If `sfp` has already been freed, this
> results in a slab-use-after-free error.
>
> Fix:
> The `kref_put(&sfp->f_ref, sg_remove_sfp)` call is moved after unlocking
> the `open_rel_lock` mutex. This ensures:
> - No references to `sfp` or `sdp` occur after the reference count is
> decremented.
> - Cleanup functions such as sg_remove_sfp and sg_remove_sfp_usercontext
> can safely execute without impacting the mutex handling in `sg_release`.
>
> The fix has been tested and validated by syzbot. This patch closes the bug
> reported at the following syzkaller link and ensures proper sequencing of
> resource cleanup and mutex operations, eliminating the risk of
> use-after-free errors in `sg_release`.
>
> Reported-by: syzbot+7efb5850a17ba6ce098b@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=7efb5850a17ba6ce098b
> Tested-by: syzbot+7efb5850a17ba6ce098b@syzkaller.appspotmail.com
> Fixes: cc833acbee9d ("sg: O_EXCL and other lock handling ")
> Signed-off-by: Suraj Sonawane <surajsonawane0215@gmail.com>
> ---
> drivers/scsi/sg.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/scsi/sg.c b/drivers/scsi/sg.c
> index f86be197f..457d54171 100644
> --- a/drivers/scsi/sg.c
> +++ b/drivers/scsi/sg.c
> @@ -393,7 +393,6 @@ sg_release(struct inode *inode, struct file *filp)
>
> mutex_lock(&sdp->open_rel_lock);
> scsi_autopm_put_device(sdp->device);
> - kref_put(&sfp->f_ref, sg_remove_sfp);
> sdp->open_cnt--;
>
> /* possibly many open()s waiting on exlude clearing, start many;
> @@ -405,6 +404,7 @@ sg_release(struct inode *inode, struct file *filp)
> wake_up_interruptible(&sdp->open_wait);
> }
> mutex_unlock(&sdp->open_rel_lock);
> + kref_put(&sfp->f_ref, sg_remove_sfp);
> return 0;
> }
>

Hello!

I wanted to follow up on the patch I submitted. I was wondering if you
had a chance to review it and if there are any comments or feedback.

Thank you for your time and consideration. I look forward to your response.

Best regards,
Suraj Sonawane