From: Suraj Sonawane surajsonawane0215@gmail.com

commit f10593ad9bc36921f623361c9e3dd96bd52d85ee upstream.

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

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.

Move the kref_put(&sfp->f_ref, sg_remove_sfp) call 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 Link: https://lore.kernel.org/r/20241120125944.88095-1-surajsonawane0215@gmail.com Reviewed-by: Bart Van Assche bvanassche@acm.org Signed-off-by: Martin K. Petersen martin.petersen@oracle.com Signed-off-by: Alva Lan alvalan9@foxmail.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 12344be14232..1946cc96c172 100644 --- a/drivers/scsi/sg.c +++ b/drivers/scsi/sg.c @@ -390,7 +390,6 @@ 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; @@ -402,6 +401,7 @@ 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; }