Message-Id: <20180523153714.28470-3-tytso@mit.edu>
Date:   Wed, 23 May 2018 11:37:14 -0400
From:   Theodore Ts'o <tytso@....edu>
To:     Ext4 Developers List <linux-ext4@...r.kernel.org>
Cc:     wen.xu@...ech.edu, jannh@...gle.com, Theodore Ts'o <tytso@....edu>
Subject: [PATCH 3/3] ext4: correctly handle a zero-length xattr with a non-zero e_value_offs

Ext4 will always create ext4 extended attributes which do not have a
value (where e_value_size is zero) with e_value_offs set to zero.  In
most places e_value_offs will not be used in a substantive way if
e_value_size is zero.

There was one exception to this, which is in ext4_xattr_set_entry(),
where if there is a maliciously crafted file system where there is an
extended attribute with e_value_offs is non-zero and e_values_size is
0, the attempt to remove this xattr will result in a negative value
getting passed to memmove, leading to the following sadness:

[   41.225365] EXT4-fs (loop0): mounted filesystem with ordered data mode. Opts: (null)
[   44.538641] BUG: unable to handle kernel paging request at ffff9ec9a3000000
[   44.538733] IP: __memmove+0x81/0x1a0
[   44.538755] PGD 1249bd067 P4D 1249bd067 PUD 1249c1067 PMD 80000001230000e1
[   44.538793] Oops: 0003 [#1] SMP PTI
[   44.538815] Modules linked in: snd_hda_codec_generic snd_hda_intel snd_hda_codec snd_hwdep snd_hda_core snd_pcm snd_timer snd soundcore i2c_piix4 mac_hid ib_iser rdma_cm iw_cm ib_cm ib_core iscsi_tcp libiscsi_tcp libiscsi scsi_transport_iscsi autofs4 btrfs zstd_decompress zstd_compress xxhash raid10 raid456 libcrc32c async_raid6_recov async_memcpy async_pq async_xor async_tx xor raid6_pq raid1 raid0 multipath linear qxl 8139too drm_kms_helper syscopyarea sysfillrect sysimgblt fb_sys_fops ttm crct10dif_pclmul drm 8139cp floppy crc32_pclmul aesni_intel aes_x86_64 crypto_simd cryptd pata_acpi glue_helper mii
[   44.539074] CPU: 0 PID: 1470 Comm: poc Not tainted 4.16.0-rc1+ #1
[   44.539104] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Ubuntu-1.8.2-1ubuntu1 04/01/2014
[   44.539147] RIP: 0010:__memmove+0x81/0x1a0
[   44.539170] RSP: 0018:ffffb84e00cb7a30 EFLAGS: 00010203
[   44.539199] RAX: ffff9ec9a15a6400 RBX: ffffb84e00cb7c38 RCX: 1fffffffffcb4c7e
[   44.539231] RDX: fffffffffffffff4 RSI: ffff9ec9a3000000 RDI: ffff9ec9a3000000
[   44.539263] RBP: ffffb84e00cb7bb0 R08: 0000000000000000 R09: ffffffff83321992
[   44.539295] R10: ffff9ec9a15a63ec R11: 0000000000000000 R12: ffff9ec9a15a6020
[   44.539328] R13: 00000000000003f4 R14: ffff9ec9a15a6400 R15: 0000000000000000
[   44.539361] FS:  00007f3628101700(0000) GS:ffff9ec9bfc00000(0000) knlGS:0000000000000000
[   44.539397] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   44.539424] CR2: ffff9ec9a3000000 CR3: 0000000138c52000 CR4: 00000000000006f0
[   44.539475] Call Trace:
[   44.539832]  ext4_xattr_set_entry+0x9e7/0xf80
[   44.539871]  ? jbd2_journal_cancel_revoke+0xbb/0xe0
[   44.539897]  ? do_get_write_access+0x318/0x400
[   44.539924]  ? kmem_cache_alloc+0xd9/0x1b0
[   44.539946]  ? jbd2_journal_get_write_access+0x54/0x60
[   44.539972]  ext4_xattr_block_set+0x212/0xea0
[   44.539998]  ? _cond_resched+0x16/0x40
[   44.540019]  ? xattr_find_entry+0x89/0x110
[   44.540041]  ext4_xattr_set_handle+0x514/0x610
[   44.540065]  ext4_xattr_set+0x7f/0x120
[   44.540090]  __vfs_removexattr+0x4d/0x60
[   44.540112]  vfs_removexattr+0x75/0xe0
[   44.540132]  removexattr+0x4d/0x80
[   44.540152]  ? kmem_cache_alloc+0xd9/0x1b0
[   44.540174]  ? _cond_resched+0x16/0x40
[   44.540194]  ? kmem_cache_alloc+0xd9/0x1b0
[   44.540217]  ? _cond_resched+0x16/0x40
[   44.540238]  ? __mnt_want_write+0x54/0x60
[   44.540259]  ? mnt_want_write+0x28/0x50
[   44.540279]  path_removexattr+0x91/0xb0
[   44.540300]  SyS_removexattr+0xf/0x20
[   44.540322]  do_syscall_64+0x71/0x120
[   44.540344]  entry_SYSCALL_64_after_hwframe+0x21/0x86
[   44.541387] RIP: 0033:0x7f3627c221c7
[   44.542304] RSP: 002b:00007ffe569d7248 EFLAGS: 00000206 ORIG_RAX: 00000000000000c5
[   44.543244] RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f3627c221c7
[   44.544186] RDX: 0000000000000071 RSI: 0000000000401489 RDI: 000000000233f0c0
[   44.545111] RBP: 00007ffe569d73b0 R08: 000000000233f0a0 R09: 0000000000000000
[   44.546025] R10: 0000000000000595 R11: 0000000000000206 R12: 0000000000400c20
[   44.546935] R13: 00007ffe569d74b0 R14: 0000000000000000 R15: 0000000000000000
[   44.547829] Code: 08 4c 89 4f 10 4c 89 47 18 48 8d 7f 20 73 d4 48 83 c2 20 e9 a2 00 00 00 66 90 48 89 d1 4c 8b 5c 16 f8 4c 8d 54 17 f8 48 c1 e9 03 <f3> 48 a5 4d 89 1a e9 0c 01 00 00 0f 1f 40 00 48 89 d1 4c 8b 1e
[   44.549629] RIP: __memmove+0x81/0x1a0 RSP: ffffb84e00cb7a30
[   44.550479] CR2: ffff9ec9a3000000
[   44.551304] ---[ end trace 71ac2ebfa045556f ]---

https://bugzilla.kernel.org/show_bug.cgi?id=199347

Reported-by: "Xu, Wen" <wen.xu@...ech.edu>
Signed-off-by: Theodore Ts'o <tytso@....edu>
Cc: stable@...nel.org
Fixes: dec214d00e0d7 ("ext4: xattr inode deduplication")
---
 fs/ext4/xattr.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/ext4/xattr.c b/fs/ext4/xattr.c
index 499cb4b1fbd2..fc4ced59c565 100644
--- a/fs/ext4/xattr.c
+++ b/fs/ext4/xattr.c
@@ -1688,7 +1688,7 @@ static int ext4_xattr_set_entry(struct ext4_xattr_info *i,
 
 	/* No failures allowed past this point. */
 
-	if (!s->not_found && here->e_value_offs) {
+	if (!s->not_found && here->e_value_size && here->e_value_offs) {
 		/* Remove the old value. */
 		void *first_val = s->base + min_offs;
 		size_t offs = le16_to_cpu(here->e_value_offs);
-- 
2.16.1.72.g5be1f00a9a

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