On Mon, Nov 06, 2017 at 10:43:18AM +0800, Qu Wenruo wrote:
> [BUG]
> Kernel panic when mounting with "-o compress" mount option.
> KASAN will report like:
> ------
> ==================================================================
> BUG: KASAN: wild-memory-access in strncmp+0x31/0xc0
> Read of size 1 at addr d86735fce994f800 by task mount/662
> ...
> Call Trace:
> dump_stack+0xe3/0x175
> kasan_report+0x163/0x370
> __asan_load1+0x47/0x50
> strncmp+0x31/0xc0
> btrfs_compress_str2level+0x20/0x70 [btrfs]
> btrfs_parse_options+0xff4/0x1870 [btrfs]
> open_ctree+0x2679/0x49f0 [btrfs]
> btrfs_mount+0x1b7f/0x1d30 [btrfs]
> mount_fs+0x49/0x190
> vfs_kern_mount.part.29+0xba/0x280
> vfs_kern_mount+0x13/0x20
> btrfs_mount+0x31e/0x1d30 [btrfs]
> mount_fs+0x49/0x190
> vfs_kern_mount.part.29+0xba/0x280
> do_mount+0xaad/0x1a00
> SyS_mount+0x98/0xe0
> entry_SYSCALL_64_fastpath+0x1f/0xbe
> ------
>
> [Cause]
> For 'compress' and 'compress_force' options, its token doesn't expect
> any parameter so its args[0] contains uninitialized data.
> Accessing args[0] will cause above wild memory access.
>
> [Fix]
> For Opt_compress and Opt_compress_force, set compression level to
> Z_DEFAULT_COMPRESSION manually.
>
> NOTE: Don't set zlib compression level to 0 by default, which means no
> compression.
But we never set the level to 0 at the point the compression actually
happens. See zlib.c:zlib_set_level, if level is 0 then the level
passed to zlib is 3. Z_DEFAULT_COMPRESSION is upstream zlib level 6,
which is slower, we need zlib to stay in the real-time numbers.
> @@ -507,8 +508,19 @@ int btrfs_parse_options(struct btrfs_fs_info *info, char
> *options,
> token == Opt_compress_force ||
> strncmp(args[0].from, "zlib", 4) == 0) {
> compress_type = "zlib";
> +
> info->compress_type = BTRFS_COMPRESS_ZLIB;
> - info->compress_level =
> + /*
> + * args[0] contains uninitialized data since
> + * for these tokens we don't expect any
> + * parameter.
> + */
> + if (token == Opt_compress ||
> + token == Opt_compress_force)
> + info->compress_level =
> + Z_DEFAULT_COMPRESSION;
> + else
> + info->compress_level =
> btrfs_compress_str2level(args[0].from);
At least this will not screw up the levels, anything that's not
recognized will become the default.
> btrfs_set_opt(info->mount_opt, COMPRESS);
> btrfs_clear_opt(info->mount_opt, NODATACOW);
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