Messages in this thread | | | Date | Wed, 22 Feb 2023 15:29:49 +0000 | Subject | Re: [PATCH] ext4: fix another off-by-one fsmap error on 1k block filesystems | From | Tudor Ambarus <> |
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Hi, Darrick,
On 2/17/23 08:10, Tudor Ambarus wrote: > Hi, Darrick, > > On 2/16/23 18:55, Darrick J. Wong wrote: >> From: Darrick J. Wong <djwong@kernel.org> >> >> Apparently syzbot figured out that issuing this FSMAP call: >> >> struct fsmap_head cmd = { >> .fmh_count = ...; >> .fmh_keys = { >> { .fmr_device = /* ext4 dev */, .fmr_physical = 0, }, >> { .fmr_device = /* ext4 dev */, .fmr_physical = 0, }, >> }, >> ... >> }; >> ret = ioctl(fd, FS_IOC_GETFSMAP, &cmd); >> >> Produces this crash if the underlying filesystem is a 1k-block ext4 >> filesystem: >> >> kernel BUG at fs/ext4/ext4.h:3331! >> invalid opcode: 0000 [#1] PREEMPT SMP >> CPU: 3 PID: 3227965 Comm: xfs_io Tainted: G W O >> 6.2.0-rc8-achx >> Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.15.0-1 >> 04/01/2014 >> RIP: 0010:ext4_mb_load_buddy_gfp+0x47c/0x570 [ext4] >> RSP: 0018:ffffc90007c03998 EFLAGS: 00010246 >> RAX: ffff888004978000 RBX: ffffc90007c03a20 RCX: ffff888041618000 >> RDX: 0000000000000000 RSI: 00000000000005a4 RDI: ffffffffa0c99b11 >> RBP: ffff888012330000 R08: ffffffffa0c2b7d0 R09: 0000000000000400 >> R10: ffffc90007c03950 R11: 0000000000000000 R12: 0000000000000001 >> R13: 00000000ffffffff R14: 0000000000000c40 R15: ffff88802678c398 >> FS: 00007fdf2020c880(0000) GS:ffff88807e100000(0000) >> knlGS:0000000000000000 >> CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 >> CR2: 00007ffd318a5fe8 CR3: 000000007f80f001 CR4: 00000000001706e0 >> Call Trace: >> <TASK> >> ext4_mballoc_query_range+0x4b/0x210 [ext4 >> dfa189daddffe8fecd3cdfd00564e0f265a8ab80] >> ext4_getfsmap_datadev+0x713/0x890 [ext4 >> dfa189daddffe8fecd3cdfd00564e0f265a8ab80] >> ext4_getfsmap+0x2b7/0x330 [ext4 >> dfa189daddffe8fecd3cdfd00564e0f265a8ab80] >> ext4_ioc_getfsmap+0x153/0x2b0 [ext4 >> dfa189daddffe8fecd3cdfd00564e0f265a8ab80] >> __ext4_ioctl+0x2a7/0x17e0 [ext4 >> dfa189daddffe8fecd3cdfd00564e0f265a8ab80] >> __x64_sys_ioctl+0x82/0xa0 >> do_syscall_64+0x2b/0x80 >> entry_SYSCALL_64_after_hwframe+0x46/0xb0 >> RIP: 0033:0x7fdf20558aff >> RSP: 002b:00007ffd318a9e30 EFLAGS: 00000246 ORIG_RAX: 0000000000000010 >> RAX: ffffffffffffffda RBX: 00000000000200c0 RCX: 00007fdf20558aff >> RDX: 00007fdf1feb2010 RSI: 00000000c0c0583b RDI: 0000000000000003 >> RBP: 00005625c0634be0 R08: 00005625c0634c40 R09: 0000000000000001 >> R10: 0000000000000000 R11: 0000000000000246 R12: 00007fdf1feb2010 >> R13: 00005625be70d994 R14: 0000000000000800 R15: 0000000000000000 >> >> For GETFSMAP calls, the caller selects a physical block device by >> writing its block number into fsmap_head.fmh_keys[01].fmr_device. >> To query mappings for a subrange of the device, the starting byte of the >> range is written to fsmap_head.fmh_keys[0].fmr_physical and the last >> byte of the range goes in fsmap_head.fmh_keys[1].fmr_physical. >> >> IOWs, to query what mappings overlap with bytes 3-14 of /dev/sda, you'd >> set the inputs as follows: >> >> fmh_keys[0] = { .fmr_device = major(8, 0), .fmr_physical = 3}, >> fmh_keys[1] = { .fmr_device = major(8, 0), .fmr_physical = 14}, >> >> Which would return you whatever is mapped in the 12 bytes starting at >> physical offset 3. >> >> The crash is due to insufficient range validation of keys[1] in >> ext4_getfsmap_datadev. On 1k-block filesystems, block 0 is not part of >> the filesystem, which means that s_first_data_block is nonzero. >> ext4_get_group_no_and_offset subtracts this quantity from the blocknr >> argument before cracking it into a group number and a block number >> within a group. IOWs, block group 0 spans blocks 1-8192 (1-based) >> instead of 0-8191 (0-based) like what happens with larger blocksizes. >> >> The net result of this encoding is that blocknr < s_first_data_block is >> not a valid input to this function. The end_fsb variable is set from >> the keys that are copied from userspace, which means that in the above >> example, its value is zero. That leads to an underflow here: >> >> blocknr = blocknr - le32_to_cpu(es->s_first_data_block); >> >> The division then operates on -1: >> >> offset = do_div(blocknr, EXT4_BLOCKS_PER_GROUP(sb)) >> >> EXT4_SB(sb)->s_cluster_bits; >> >> Leaving an impossibly large group number (2^32-1) in blocknr. >> ext4_getfsmap_check_keys checked that keys[0].fmr_physical and >> keys[1].fmr_physical are in increasing order, but >> ext4_getfsmap_datadev adjusts keys[0].fmr_physical to be at least >> s_first_data_block. This implies that we have to check it again after >> the adjustment, which is the piece that I forgot. >> >> Fixes: 4a4956249dac ("ext4: fix off-by-one fsmap error on 1k block >> filesystems") >> Link: >> https://syzkaller.appspot.com/bug?id=79d5768e9bfe362911ac1a5057a36fc6b5c30002 >> Signed-off-by: Darrick J. Wong <djwong@kernel.org> >> --- >> fs/ext4/fsmap.c | 2 ++ >> 1 file changed, 2 insertions(+) >> >> diff --git a/fs/ext4/fsmap.c b/fs/ext4/fsmap.c >> index 4493ef0c715e..cdf9bfe10137 100644 >> --- a/fs/ext4/fsmap.c >> +++ b/fs/ext4/fsmap.c >> @@ -486,6 +486,8 @@ static int ext4_getfsmap_datadev(struct >> super_block *sb, >> keys[0].fmr_physical = bofs; >> if (keys[1].fmr_physical >= eofs) >> keys[1].fmr_physical = eofs - 1; >> + if (keys[1].fmr_physical < keys[0].fmr_physical) >> + return 0; > > This is an indirect implication, we can be more straightforward. Also we > should stop the execution when high_key->fmr_physical == bofs. So maybe: > > --- a/fs/ext4/fsmap.c > +++ b/fs/ext4/fsmap.c > @@ -479,6 +479,8 @@ static int ext4_getfsmap_datadev(struct super_block > *sb, > int error = 0; > > bofs = le32_to_cpu(sbi->s_es->s_first_data_block); > + if (keys[1].fmr_physical <= bofs) > + return 0; > eofs = ext4_blocks_count(sbi->s_es); > if (keys[0].fmr_physical >= eofs) > return 0;
Just wanted to let you know that I sent this patch together with two others at: https://lore.kernel.org/linux-ext4/20230222131211.3898066-1-tudor.ambarus@linaro.org/T/
Cheers, ta
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