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 is between a concurrent open() and rename() of the same path. __d_alloc() only stores the name itself and its terminating NUL, so the rest of the inline buffer (d_shortname, DNAME_INLINE_LEN bytes) is left uninitialized. copy_name(), called from rename(), copies that buffer as a whole, so the uninitialized tail is propagated into the dentry that is being moved. Meanwhile __d_lookup_rcu(), called from open(), is an optimistic lockless lookup: it checks d_name.hash_len first and leaves the seqcount retry to its caller, so it can end up comparing against a dentry whose name a rename is rewriting in place, using a stale (longer) length. The comparison then runs past the terminating NUL and reads bytes of the uninitialized tail, which KMSAN reports. The read is harmless by design: it stays inside the buffer, the name is still NUL-terminated, and the result is thrown away by the seqcount retry. It is not specific to KMSAN either - with CONFIG_DCACHE_WORD_ACCESS enabled the very same bytes are read by read_word_at_a_time(), which is __no_sanitize_or_inline and therefore invisible to KMSAN. KMSAN builds only see the instrumented byte-at-a-time dentry_string_cmp() because CONFIG_DCACHE_WORD_ACCESS is disabled when KMSAN is enabled on x86: commit 7cf8f44a5a1c ("x86: fs: kmsan: disable CONFIG_DCACHE_WORD_ACCESS") Zeroing the inline buffer would hide the report, but it would add a memset() to a hot allocation path just to initialize bytes that are never used as part of a name. Instead, tell KMSAN the inline buffer is initialized: kmsan_unpoison_memory() compiles to nothing unless CONFIG_KMSAN is set, and doing it at allocation time is enough for every dentry, because copy_name() and swap_names() copy the whole buffer and thus propagate its shadow. Reported-by: syzbot+7ff3adde89dd795ad4c4@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=7ff3adde89dd795ad4c4 Signed-off-by: Drif Abdelmalek Mohamed Said Changes in v3: - Annotate for KMSAN instead of zeroing, as suggested in review: the read is harmless, so unpoison the inline buffer in __d_alloc() with kmsan_unpoison_memory() (a no-op unless CONFIG_KMSAN) rather than adding a memset() to the dentry allocation path. - Document why only KMSAN builds report this at all: with CONFIG_DCACHE_WORD_ACCESS the same read goes through read_word_at_a_time(), which KMSAN does not instrument. - Rewrite the commit message; the previous one had several truncated lines. --- fs/dcache.c | 4 ++++ 1 file changed, 4 insertions(+) diff --git a/fs/dcache.c b/fs/dcache.c index 1b1a81f10da6..a66be85f9d01 100644 --- a/fs/dcache.c +++ b/fs/dcache.c @@ -1916,6 +1916,10 @@ static struct dentry *__d_alloc(struct super_block *sb, const struct qstr *name) * be overwriting an internal NUL character */ dentry->d_shortname.string[DNAME_INLINE_LEN-1] = 0; + + /* Racy __d_lookup_rcu() walk may read past the NUL; harmless */ + kmsan_unpoison_memory(dentry->d_shortname.string, DNAME_INLINE_LEN); + if (unlikely(!name)) { name = &slash_name; dname = dentry->d_shortname.string; -- 2.43.0