Re: [syzbot] [integrity?] [lsm?] KMSAN: uninit-value in ima_add_template_entry

[Gao Xiang]

On 2024/3/3 22:54, Tetsuo Handa wrote:
> On 2024/02/20 19:40, Roberto Sassu wrote:
> > On Mon, 2024-02-19 at 22:41 -0800, syzbot wrote:
> > > Hello,
> > >
> > > syzbot found the following issue on:
> > >
> > > HEAD commit:    4f5e5092fdbf Merge tag 'net-6.8-rc5' of git://git.kernel.o..
> > > git tree:       upstream
> > > console output: https://syzkaller.appspot.com/x/log.txt?x=135ba81c180000
> > > kernel config:  https://syzkaller.appspot.com/x/.config?x=e3dd779fba027968
> > > dashboard link: https://syzkaller.appspot.com/bug?extid=7bc44a489f0ef0670bd5
> > > compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
>
> > I would add the VFS people in CC, in case they have some ideas.
>
> This is an erofs bug. Since the filesystem image in the reproducer
> is crafted, decompression generates bogus result and
> z_erofs_transform_plain() misbehaves.

Yes, thanks for looking into this.  It seems it was introduced by
a new commit 1ca01520148a this cycle, let me find more clues and
make a fix for this.

Thanks,
Gao Xiang

> You can obtain a single-threaded reproducer from
> https://syzkaller.appspot.com/x/repro.c?x=1256096a180000 with below diff.
>
> ----------------------------------------
> --- old/1256096a180000.c
> +++ new/1256096a180000.c
> @@ -676,6 +676,6 @@
>     syscall(__NR_mmap, /*addr=*/0x21000000ul, /*len=*/0x1000ul, /*prot=*/0ul,
>             /*flags=MAP_FIXED|MAP_ANONYMOUS|MAP_PRIVATE*/ 0x32ul, /*fd=*/-1,
>             /*offset=*/0ul);
> -  loop();
> +  execute_one();
>     return 0;
>   }
> ----------------------------------------
>
> With CONFIG_EROFS_FS_DEBUG=y, the reproducer hits DBG_BUGON().
> With debug printk() shown below, you can get output shown below.
>
> ----------------------------------------
> diff --git a/fs/erofs/decompressor.c b/fs/erofs/decompressor.c
> index d4cee95af14c..f221133a0731 100644
> --- a/fs/erofs/decompressor.c
> +++ b/fs/erofs/decompressor.c
> @@ -323,7 +323,11 @@ static int z_erofs_transform_plain(struct z_erofs_decompress_req *rq,
>   	unsigned int cur = 0, ni = 0, no, pi, po, insz, cnt;
>   	u8 *kin;
>   
> -	DBG_BUGON(rq->outputsize > rq->inputsize);
> +	if (rq->outputsize > rq->inputsize) {
> +		pr_err("rq->inputsize=%u rq->outputsize=%u\n", rq->inputsize, rq->outputsize);
> +		pr_err("rq->pageofs_in=%u rq->pageofs_out=%u\n", rq->pageofs_in, rq->pageofs_out);
> +		pr_err("nrpages_in=%u nrpages_out=%u\n", nrpages_in, nrpages_out);
> +	}
>   	if (rq->alg == Z_EROFS_COMPRESSION_INTERLACED) {
>   		cur = bs - (rq->pageofs_out & (bs - 1));
>   		pi = (rq->pageofs_in + rq->inputsize - cur) & ~PAGE_MASK;
> @@ -352,7 +356,8 @@ static int z_erofs_transform_plain(struct z_erofs_decompress_req *rq,
>   		do {
>   			no = (rq->pageofs_out + cur + pi) >> PAGE_SHIFT;
>   			po = (rq->pageofs_out + cur + pi) & ~PAGE_MASK;
> -			DBG_BUGON(no >= nrpages_out);
> +			if (no >= nrpages_out)
> +				pr_err("no=%u nrpages_out=%u\n", no, nrpages_out);
>   			cnt = min(insz - pi, PAGE_SIZE - po);
>   			if (rq->out[no] == rq->in[ni]) {
>   				memmove(kin + po,
> @@ -366,7 +371,8 @@ static int z_erofs_transform_plain(struct z_erofs_decompress_req *rq,
>   		} while (pi < insz);
>   		kunmap_local(kin);
>   	}
> -	DBG_BUGON(ni > nrpages_in);
> +	if (ni > nrpages_in)
> +		pr_err("ni=%u nrpages_in=%u\n", ni, nrpages_in);
>   	return 0;
>   }
>   
> ----------------------------------------
>
> ----------------------------------------
> [  138.991810][ T2983] loop0: detected capacity change from 0 to 16
> [  139.804002][ T2983] erofs: (device loop0): mounted with root inode @ nid 36.
> [  139.810464][   T87] erofs: rq->inputsize=4096 rq->outputsize=8194
> [  139.821540][   T87] erofs: rq->pageofs_in=0 rq->pageofs_out=0
> [  139.824347][   T87] erofs: nrpages_in=1 nrpages_out=3
> [  139.827008][   T87] erofs: ni=3 nrpages_in=1
> [  139.873777][ T2983] =====================================================
> [  139.881268][ T2983] BUG: KMSAN: uninit-value in ima_add_template_entry+0x626/0xa80
> ----------------------------------------
>
> #syz set subsystems: erofs