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 --- fs/dcache.c | 15 ++++++++++----- 1 file changed, 10 insertions(+), 5 deletions(-) diff --git a/fs/dcache.c b/fs/dcache.c index 1b1a81f10da6..2de1bc76dc8e 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; + memcpy(dentry->d_shortname.string, 0, DNAME_INLINE_LEN); if (unlikely(!name)) { name = &slash_name; dname = dentry->d_shortname.string; -- 2.43.0