[PATCH v2] dcache: fully initialize the inline name in __d_alloc()

[Drif Abdelmalek Mohamed Said <drifabdelmalekmohamedsaid@gmail.com>]

syzbot reported:

    BUG: KMSAN: uninit-value in dentry_string_cmp fs/dcache.c:291 [inline]
    BUG: KMSAN: uninit-value in dentry_cmp fs/dcache.c:322 [inline]
    BUG: KMSAN: uninit-value in __d_lookup_rcu+0x37d/0x5e0 fs/dcache.c:2522

     dentry_string_cmp fs/dcache.c:291 [inline]
     dentry_cmp fs/dcache.c:322 [inline]
     __d_lookup_rcu+0x37d/0x5e0 fs/dcache.c:2522
     lookup_fast+0x194/0xa40 fs/namei.c:1854
     lookup_fast_for_open fs/namei.c:4545 [inline]
     open_last_lookups fs/namei.c:4579 [inline]
     path_openat+0x9ef/0x6540 fs/namei.c:4856
     do_file_open+0x2aa/0x680 fs/namei.c:4888
     do_sys_openat2+0x17c/0x390 fs/open.c:1395
     do_sys_open fs/open.c:1401 [inline]
     __do_sys_openat fs/open.c:1417 [inline]
     __se_sys_openat fs/open.c:1412 [inline]
     __x64_sys_openat+0x240/0x300 fs/open.c:1412
     x64_sys_call+0x2445/0x3ea0 arch/x86/include/generated/asm/syscalls_64.h:258
     do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
     do_syscall_64+0x15d/0x3c0 arch/x86/entry/syscall_64.c:94
     entry_SYSCALL_64_after_hwframe+0x77/0x7f

Uninit was stored to memory at:
     copy_name fs/dcache.c:3031 [inline]
     __d_move+0xd29/0x21f0 fs/dcache.c:3099
     d_move+0x71/0xf0 fs/dcache.c:3147
     vfs_rename+0x2619/0x2770 fs/namei.c:6085
     filename_renameat2+0xa59/0x1230 fs/namei.c:6188
     __do_sys_rename fs/namei.c:6232 [inline]
     __se_sys_rename+0xc5/0x5c0 fs/namei.c:6228
     __x64_sys_rename+0x78/0xb0 fs/namei.c:6228
     x64_sys_call+0x329/0x3ea0 arch/x86/include/generated/asm/syscalls_64.h:83
     do_syscall_x64 arch/x86/entry/syscall_64.c:63
     do_syscall_64+0x15d/0x3c0 arch/x86/entry/syscall_64.c:94
     entry_SYSCALL_64_after_hwframe+0x77/0x7f

Uninit was created at:
     slab_post_alloc_hook mm/slub.c:4617 [inline]
     slab_alloc_node mm/slub.c:4939 [inline]
     kmem_cache_alloc_lru_noprof+0x376/0x1230 mm/s
     __d_alloc+0x52/0x9f0 fs/dcache.c:1902
     d_alloc+0x57/0x300 fs/dcache.c:1981
     lookup_one_qstr_excl+0x19d/0x7a0 fs/namei.c:1806
     __start_renaming+0x341/0x850 fs/namei.c:3888
     filename_renameat2+0x625/0x1230 fs/namei.c:6163
     __do_sys_rename fs/namei.c:6232 [inline]
     __se_sys_rename+0xc5/0x5c0 fs/namei.c:6228
     __x64_sys_rename+0x78/0xb0 fs/namei.c:6228
     x64_sys_call+0x329/0x3ea0 arch/x86/include/generated/asm/syscalls_64.h:83
     do_syscall_x64 arch/x86/entry/syscall_64.c:63
     do_syscall_64+0x15d/0x3c0 arch/x86/entry/syscall_64.c:94
     entry_SYSCALL_64_after_hwframe+0x77/0x7f

The race happens between a concurrent open() and rename() of the same
path.  __d_alloc() initializes only the name itsel
of the inline buffer; the tail in between is left uninitialized.
copy_name(), called from rename(), copies the enti
including that uninitialized tail - into the moved dentry.  Meanwhile
__d_lookup_rcu(), called from open(), is an optimi
it checks d_name.hash_len first and leaves the seqcount retry to its
caller, so a lookup of the old (longer) name racin
compares with a stale length.  While copy_name() is rewriting the name
in place, the walker can transiently step past the
terminating NUL and read bytes from the uninitialized tail, which KMSAN
reports.

The race is benign by design - the read stays in bounds and the lookup
result is discarded by the seqcount retry - but th
is real.  Fix it by zeroing the entire inline buffer in __d_alloc(),
so every byte a racy walker can touch is defined;
past a mid-rewrite name now simply stops at a NUL.

Reported-by: syzbot+7ff3adde89dd795ad4c4@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=7ff3adde89dd795ad4c4
Signed-off-by: Drif Abdelmalek Mohamed Said <drifabdelmalekmohamedsaid@gmail.com>
---
v2:
  - Screwed up by using memcpy in v1, fixed it here by using memset, only change
 fs/dcache.c | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/fs/dcache.c b/fs/dcache.c
index 1b1a81f10da6..730571e92af8 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -1910,12 +1910,17 @@ static struct dentry *__d_alloc(struct super_block *sb, const struct qstr *name)
 		return NULL;
 
 	/*
-	 * We guarantee that the inline name is always NUL-terminated.
-	 * This way the memcpy() done by the name switching in rename
-	 * will still always have a NUL at the end, even if we might
-	 * be overwriting an internal NUL character
+	 * Fully initialize the inline name buffer.  copy_name() and
+	 * swap_names() copy d_shortname in its entirety, so any
+	 * uninitialized tail would propagate to the other dentry, and
+	 * __d_lookup_rcu() may transiently read any byte of the inline
+	 * name while rename() rewrites it in place.
+	 *
+	 * This also keeps the inline name NUL-terminated: the name
+	 * switching in rename will still always have a NUL at the end,
+	 * even if we might be overwriting an internal NUL character.
 	 */
-	dentry->d_shortname.string[DNAME_INLINE_LEN-1] = 0;
+	memset(dentry->d_shortname.string, 0, DNAME_INLINE_LEN);
 	if (unlikely(!name)) {
 		name = &slash_name;
 		dname = dentry->d_shortname.string;
-- 
2.43.0