On 22/06/16 10:13AM, Baokun Li wrote: > Hulk Robot reported a issue: > ================================================================== > BUG: KASAN: use-after-free in ext4_xattr_set_entry+0x18ab/0x3500 > Write of size 4105 at addr ffff8881675ef5f4 by task syz-executor.0/7092 > > CPU: 1 PID: 7092 Comm: syz-executor.0 Not tainted 4.19.90-dirty #17 > Call Trace: > [...] > memcpy+0x34/0x50 mm/kasan/kasan.c:303 > ext4_xattr_set_entry+0x18ab/0x3500 fs/ext4/xattr.c:1747 > ext4_xattr_ibody_inline_set+0x86/0x2a0 fs/ext4/xattr.c:2205 > ext4_xattr_set_handle+0x940/0x1300 fs/ext4/xattr.c:2386 > ext4_xattr_set+0x1da/0x300 fs/ext4/xattr.c:2498 > __vfs_setxattr+0x112/0x170 fs/xattr.c:149 > __vfs_setxattr_noperm+0x11b/0x2a0 fs/xattr.c:180 > __vfs_setxattr_locked+0x17b/0x250 fs/xattr.c:238 > vfs_setxattr+0xed/0x270 fs/xattr.c:255 > setxattr+0x235/0x330 fs/xattr.c:520 > path_setxattr+0x176/0x190 fs/xattr.c:539 > __do_sys_lsetxattr fs/xattr.c:561 [inline] > __se_sys_lsetxattr fs/xattr.c:557 [inline] > __x64_sys_lsetxattr+0xc2/0x160 fs/xattr.c:557 > do_syscall_64+0xdf/0x530 arch/x86/entry/common.c:298 > entry_SYSCALL_64_after_hwframe+0x44/0xa9 > RIP: 0033:0x459fe9 > RSP: 002b:00007fa5e54b4c08 EFLAGS: 00000246 ORIG_RAX: 00000000000000bd > RAX: ffffffffffffffda RBX: 000000000051bf60 RCX: 0000000000459fe9 > RDX: 00000000200003c0 RSI: 0000000020000180 RDI: 0000000020000140 > RBP: 000000000051bf60 R08: 0000000000000001 R09: 0000000000000000 > R10: 0000000000001009 R11: 0000000000000246 R12: 0000000000000000 > R13: 00007ffc73c93fc0 R14: 000000000051bf60 R15: 00007fa5e54b4d80 > [...] > ================================================================== > > Above issue may happen as follows: > ------------------------------------- > ext4_xattr_set > ext4_xattr_set_handle > ext4_xattr_ibody_find > >> s->end < s->base > >> no EXT4_STATE_XATTR > >> xattr_check_inode is not executed > ext4_xattr_ibody_set > ext4_xattr_set_entry > >> size_t min_offs = s->end - s->base > >> UAF in memcpy > > we can easily reproduce this problem with the following commands: > mkfs.ext4 -F /dev/sda > mount -o debug_want_extra_isize=128 /dev/sda /mnt > touch /mnt/file > setfattr -n user.cat -v `seq -s z 4096|tr -d '[:digit:]'` /mnt/file Thanks for sharing the test case. Indeed this results into UAF. > > In ext4_xattr_ibody_find, we have the following assignment logic: > header = IHDR(inode, raw_inode) > = raw_inode + EXT4_GOOD_OLD_INODE_SIZE + i_extra_isize > is->s.base = IFIRST(header) > = header + sizeof(struct ext4_xattr_ibody_header) > is->s.end = raw_inode + s_inode_size > > In ext4_xattr_set_entry > min_offs = s->end - s->base > = s_inode_size - EXT4_GOOD_OLD_INODE_SIZE - i_extra_isize - > sizeof(struct ext4_xattr_ibody_header) > last = s->first > free = min_offs - ((void *)last - s->base) - sizeof(__u32) > = s_inode_size - EXT4_GOOD_OLD_INODE_SIZE - i_extra_isize - > sizeof(struct ext4_xattr_ibody_header) - sizeof(__u32) > > In the calculation formula, all values except s_inode_size and > i_extra_size are fixed values. When i_extra_size is the maximum value > s_inode_size - EXT4_GOOD_OLD_INODE_SIZE, min_offs is -4 and free is -8. > The value overflows. As a result, the preceding issue is triggered when > memcpy is executed. > > Therefore, when finding xattr or setting xattr, check whether > there is space for storing xattr in the inode to resolve this issue. Sounds right. Thanks for fixing it and providing detailed analysis. Feel free to add - Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@xxxxxxxxx> > > Reported-by: Hulk Robot <hulkci@xxxxxxxxxx> > Signed-off-by: Baokun Li <libaokun1@xxxxxxxxxx> > --- > fs/ext4/xattr.c | 6 ++++-- > 1 file changed, 4 insertions(+), 2 deletions(-) > > diff --git a/fs/ext4/xattr.c b/fs/ext4/xattr.c > index 042325349098..c3c3194f3ee1 100644 > --- a/fs/ext4/xattr.c > +++ b/fs/ext4/xattr.c > @@ -2176,8 +2176,9 @@ int ext4_xattr_ibody_find(struct inode *inode, struct ext4_xattr_info *i, > struct ext4_inode *raw_inode; > int error; > > - if (EXT4_I(inode)->i_extra_isize == 0) > + if (!EXT4_INODE_HAS_XATTR_SPACE(inode)) > return 0; > + > raw_inode = ext4_raw_inode(&is->iloc); > header = IHDR(inode, raw_inode); > is->s.base = is->s.first = IFIRST(header); > @@ -2205,8 +2206,9 @@ int ext4_xattr_ibody_set(handle_t *handle, struct inode *inode, > struct ext4_xattr_search *s = &is->s; > int error; > > - if (EXT4_I(inode)->i_extra_isize == 0) > + if (!EXT4_INODE_HAS_XATTR_SPACE(inode)) > return -ENOSPC; > + > error = ext4_xattr_set_entry(i, s, handle, inode, false /* is_block */); > if (error) > return error; > -- > 2.31.1 >
