Re: [syzbot] KMSAN: uninit-value in pagecache_write

[Alexander Potapenko <glider@google.com>]

On Mon, Nov 7, 2022 at 10:10 AM syzbot
<syzbot+9767be679ef5016b6082@syzkaller.appspotmail.com> wrote:
>
> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit:    968c2729e576 x86: kmsan: fix comment in kmsan_shadow.c
> git tree:       https://github.com/google/kmsan.git master
> console output: https://syzkaller.appspot.com/x/log.txt?x=11d01ad6880000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=131312b26465c190
> dashboard link: https://syzkaller.appspot.com/bug?extid=9767be679ef5016b6082
> compiler:       clang version 15.0.0 (https://github.com/llvm/llvm-project.git 610139d2d9ce6746b3c617fb3e2f7886272d26ff), GNU ld (GNU Binutils for Debian) 2.35.2
> userspace arch: i386
>
> Unfortunately, I don't have any reproducer for this issue yet.
>
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/c78ce21b953f/disk-968c2729.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/22868d826804/vmlinux-968c2729.xz
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+9767be679ef5016b6082@syzkaller.appspotmail.com
>
> =====================================================
> BUG: KMSAN: uninit-value in pagecache_write+0x655/0x720 fs/ext4/verity.c:91
>  pagecache_write+0x655/0x720 fs/ext4/verity.c:91
>  ext4_write_merkle_tree_block+0x84/0xa0 fs/ext4/verity.c:389
>  build_merkle_tree_level+0x972/0x1250 fs/verity/enable.c:121
>  build_merkle_tree fs/verity/enable.c:182 [inline]
>  enable_verity+0xede/0x1920 fs/verity/enable.c:268
>  fsverity_ioctl_enable+0x895/0xab0 fs/verity/enable.c:392
>  __ext4_ioctl fs/ext4/ioctl.c:1572 [inline]
>  ext4_ioctl+0x26dd/0x8c50 fs/ext4/ioctl.c:1606
>  ext4_compat_ioctl+0x702/0x800 fs/ext4/ioctl.c:1682
>  __do_compat_sys_ioctl fs/ioctl.c:968 [inline]
>  __se_compat_sys_ioctl+0x781/0xfa0 fs/ioctl.c:910
>  __ia32_compat_sys_ioctl+0x8f/0xd0 fs/ioctl.c:910
>  do_syscall_32_irqs_on arch/x86/entry/common.c:112 [inline]
>  __do_fast_syscall_32+0xa2/0x100 arch/x86/entry/common.c:178
>  do_fast_syscall_32+0x33/0x70 arch/x86/entry/common.c:203
>  do_SYSENTER_32+0x1b/0x20 arch/x86/entry/common.c:246
>  entry_SYSENTER_compat_after_hwframe+0x70/0x82
>
> Local variable fsdata created at:
>  pagecache_write+0x21c/0x720 fs/ext4/verity.c:85
>  ext4_write_merkle_tree_block+0x84/0xa0 fs/ext4/verity.c:389
>
> CPU: 1 PID: 15121 Comm: syz-executor.3 Not tainted 6.0.0-rc5-syzkaller-48543-g968c2729e576 #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/11/2022
> =====================================================

This is identical to other reports fixed in
https://lore.kernel.org/lkml/20220915150417.722975-43-glider@google.com/
To fix the error, we need to initialize fsdata explicitly, because
aops->write_begin is not guaranteed to do so:

=============================================================================
   ext4: initialize fsdata in pagecache_write()

    When aops->write_begin() does not initialize fsdata, KMSAN reports
    an error passing the latter to aops->write_end().

    Fix this by unconditionally initializing fsdata.

    Fixes: c93d8f885809 ("ext4: add basic fs-verity support")
    Reported-by: syzbot+9767be679ef5016b6082@syzkaller.appspotmail.com
    Signed-off-by: Alexander Potapenko <glider@google.com>

diff --git a/fs/ext4/verity.c b/fs/ext4/verity.c
index 3c640bd7ecaeb..30e3b65798b50 100644
--- a/fs/ext4/verity.c
+++ b/fs/ext4/verity.c
@@ -79,7 +79,7 @@ static int pagecache_write(struct inode *inode,
const void *buf, size_t count,
                size_t n = min_t(size_t, count,
                                 PAGE_SIZE - offset_in_page(pos));
                struct page *page;
-               void *fsdata;
+               void *fsdata = NULL;
                int res;

                res = aops->write_begin(NULL, mapping, pos, n, &page, &fsdata);
=============================================================================