The syzbot fuzzer and Gerald Lee have identified a use-after-free bug in the gadgetfs driver, involving processes concurrently mounting and unmounting the gadgetfs filesystem. In particular, gadgetfs_fill_super() can race with gadgetfs_kill_sb(), causing the latter to deallocate the_device while the former is using it. The output from KASAN says, in part: BUG: KASAN: use-after-free in instrument_atomic_read_write include/linux/instrumented.h:102 [inline] BUG: KASAN: use-after-free in atomic_fetch_sub_release include/linux/atomic/atomic-instrumented.h:176 [inline] BUG: KASAN: use-after-free in __refcount_sub_and_test include/linux/refcount.h:272 [inline] BUG: KASAN: use-after-free in __refcount_dec_and_test include/linux/refcount.h:315 [inline] BUG: KASAN: use-after-free in refcount_dec_and_test include/linux/refcount.h:333 [inline] BUG: KASAN: use-after-free in put_dev drivers/usb/gadget/legacy/inode.c:159 [inline] BUG: KASAN: use-after-free in gadgetfs_kill_sb+0x33/0x100 drivers/usb/gadget/legacy/inode.c:2086 Write of size 4 at addr ffff8880276d7840 by task syz-executor126/18689 CPU: 0 PID: 18689 Comm: syz-executor126 Not tainted 6.1.0-syzkaller #0 Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/26/2022 Call Trace: <TASK> ... atomic_fetch_sub_release include/linux/atomic/atomic-instrumented.h:176 [inline] __refcount_sub_and_test include/linux/refcount.h:272 [inline] __refcount_dec_and_test include/linux/refcount.h:315 [inline] refcount_dec_and_test include/linux/refcount.h:333 [inline] put_dev drivers/usb/gadget/legacy/inode.c:159 [inline] gadgetfs_kill_sb+0x33/0x100 drivers/usb/gadget/legacy/inode.c:2086 deactivate_locked_super+0xa7/0xf0 fs/super.c:332 vfs_get_super fs/super.c:1190 [inline] get_tree_single+0xd0/0x160 fs/super.c:1207 vfs_get_tree+0x88/0x270 fs/super.c:1531 vfs_fsconfig_locked fs/fsopen.c:232 [inline] The simplest solution is to ensure that gadgetfs_fill_super() and gadgetfs_kill_sb() are serialized by making them both acquire a new mutex. Signed-off-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> Reported-and-tested-by: syzbot+33d7ad66d65044b93f16@xxxxxxxxxxxxxxxxxxxxxxxxx Reported-and-tested-by: Gerald Lee <sundaywind2004@xxxxxxxxx> Link: https://lore.kernel.org/linux-usb/CAO3qeMVzXDP-JU6v1u5Ags6Q-bb35kg3=C6d04DjzA9ffa5x1g@xxxxxxxxxxxxxx/ Fixes: e5d82a7360d1 ("vfs: Convert gadgetfs to use the new mount API") CC: <stable@xxxxxxxxxxxxxxx> --- I really don't know if the Fixes: commit above is the one which caused this bug, but it's a reasonable guess since it introduced one of the two racing paths (presumably, before this commit the routines didn't race). In any case, this patch should apply to all the currently maintained kernel versions. [as1989] drivers/usb/gadget/legacy/inode.c | 28 +++++++++++++++++++++------- 1 file changed, 21 insertions(+), 7 deletions(-) Index: usb-devel/drivers/usb/gadget/legacy/inode.c =================================================================== --- usb-devel.orig/drivers/usb/gadget/legacy/inode.c +++ usb-devel/drivers/usb/gadget/legacy/inode.c @@ -229,6 +229,7 @@ static void put_ep (struct ep_data *data */ static const char *CHIP; +static DEFINE_MUTEX(sb_mutex); /* Serialize superblock operations */ /*----------------------------------------------------------------------*/ @@ -2010,13 +2011,20 @@ gadgetfs_fill_super (struct super_block { struct inode *inode; struct dev_data *dev; + int rc; - if (the_device) - return -ESRCH; + mutex_lock(&sb_mutex); + + if (the_device) { + rc = -ESRCH; + goto Done; + } CHIP = usb_get_gadget_udc_name(); - if (!CHIP) - return -ENODEV; + if (!CHIP) { + rc = -ENODEV; + goto Done; + } /* superblock */ sb->s_blocksize = PAGE_SIZE; @@ -2053,13 +2061,17 @@ gadgetfs_fill_super (struct super_block * from binding to a controller. */ the_device = dev; - return 0; + rc = 0; + goto Done; -Enomem: + Enomem: kfree(CHIP); CHIP = NULL; + rc = -ENOMEM; - return -ENOMEM; + Done: + mutex_unlock(&sb_mutex); + return rc; } /* "mount -t gadgetfs path /dev/gadget" ends up here */ @@ -2081,6 +2093,7 @@ static int gadgetfs_init_fs_context(stru static void gadgetfs_kill_sb (struct super_block *sb) { + mutex_lock(&sb_mutex); kill_litter_super (sb); if (the_device) { put_dev (the_device); @@ -2088,6 +2101,7 @@ gadgetfs_kill_sb (struct super_block *sb } kfree(CHIP); CHIP = NULL; + mutex_unlock(&sb_mutex); } /*----------------------------------------------------------------------*/
