linux-kernel - Re: [PATCH] reiserfs: fix broken xattr handling (heap corruption, bad retval)

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Date:   Mon, 13 Aug 2018 14:39:10 -0400
From:   Jeff Mahoney <jeffm@...e.com>
To:     Jann Horn <jannh@...gle.com>, reiserfs-devel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org, Eric Biggers <ebiggers@...gle.com>,
        Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH] reiserfs: fix broken xattr handling (heap corruption, bad
 retval)

On 8/2/18 11:15 AM, Jann Horn wrote:
> This fixes the following issues:
> 
>  - When a buffer size is supplied to reiserfs_listxattr() such that each
>    individual name fits, but the concatenation of all names doesn't
>    fit, reiserfs_listxattr() overflows the supplied buffer. This leads to
>    a kernel heap overflow (verified using KASAN) followed by an
>    out-of-bounds usercopy and is therefore a security bug.
>  - When a buffer size is supplied to reiserfs_listxattr() such that a name
>    doesn't fit, -ERANGE should be returned. But reiserfs instead just
>    truncates the list of names; I have verified that if the only xattr on
>    a file has a longer name than the supplied buffer length, listxattr()
>    incorrectly returns zero.
> 
> With my patch applied, -ERANGE is returned in both cases and the memory
> corruption doesn't happen anymore.
> 
> Credit for making me clean this code up a bit goes to Al Viro, who pointed
> out that the ->actor calling convention is suboptimal and should be
> changed.
> 
> Fixes: 48b32a3553a5 ("reiserfs: use generic xattr handlers")
> Cc: stable@...r.kernel.org
> Signed-off-by: Jann Horn <jannh@...gle.com>

Acked-by: Jeff Mahoney <jeffm@...e.com>

Thanks,

-Jeff

> ---
> Triggering the bug:
> 
> root@...ian:/home/user# mount -o user_xattr reiserimg reisermount/
> root@...ian:/home/user# cd reisermount/
> root@...ian:/home/user/reisermount# touch test_file
> root@...ian:/home/user/reisermount# setfattr -n user.foo1 -v A test_file
> root@...ian:/home/user/reisermount# setfattr -n user.foo2 -v A test_file
> root@...ian:/home/user/reisermount# setfattr -n user.foo3 -v A test_file
> root@...ian:/home/user/reisermount# setfattr -n user.foo4 -v A test_file
> root@...ian:/home/user/reisermount# setfattr -n user.foo5 -v A test_file
> root@...ian:/home/user/reisermount# setfattr -n user.foo6 -v A test_file
> root@...ian:/home/user/reisermount# cat xattr_test.c
> #include <sys/types.h>
> #include <attr/xattr.h>
> #include <err.h>
> #include <stdio.h>
> #include <string.h>
> int main(int argc, char **argv) {
>   if (argc != 2) errx(1, "bad invocation");
>   char list[10];
>   int res = listxattr(argv[1], list, sizeof(list));
>   if (res == -1)
>     err(1, "listxattr failed");
>   printf("listxattr returned %d\n", res);
>   for (char *p = list; p < list+res-1; p = p + strlen(p) + 1) {
>     printf("list entry: %s\n", p);
>   }
> }
> root@...ian:/home/user/reisermount# gcc -o xattr_test xattr_test.c
> root@...ian:/home/user/reisermount# ./xattr_test test_file
> Segmentation fault
> root@...ian:/home/user/reisermount#
> 
> Result:
> 
> [  122.071318] ==================================================================
> [  122.072334] BUG: KASAN: slab-out-of-bounds in listxattr_filler+0x170/0x1b0
> [  122.073173] Write of size 9 at addr ffff8801c43b474a by task xattr_test/923
> [  122.074030]
> [  122.074223] CPU: 1 PID: 923 Comm: xattr_test Not tainted 4.18.0-rc7+ #67
> [  122.075050] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1 04/01/2014
> [  122.076107] Call Trace:
> [  122.076453]  dump_stack+0x71/0xab
> [  122.076900]  print_address_description+0x6a/0x250
> [  122.077514]  kasan_report+0x258/0x380
> [  122.077961]  ? listxattr_filler+0x170/0x1b0
> [  122.078469]  memcpy+0x34/0x50
> [  122.078894]  listxattr_filler+0x170/0x1b0
> [...]
> 
>  fs/reiserfs/xattr.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/reiserfs/xattr.c b/fs/reiserfs/xattr.c
> index ff94fad477e4..48cdfc81fe10 100644
> --- a/fs/reiserfs/xattr.c
> +++ b/fs/reiserfs/xattr.c
> @@ -792,8 +792,10 @@ static int listxattr_filler(struct dir_context *ctx, const char *name,
>  			return 0;
>  		size = namelen + 1;
>  		if (b->buf) {
> -			if (size > b->size)
> +			if (b->pos + size > b->size) {
> +				b->pos = -ERANGE;
>  				return -ERANGE;
> +			}
>  			memcpy(b->buf + b->pos, name, namelen);
>  			b->buf[b->pos + namelen] = 0;
>  		}
> 

-- 
Jeff Mahoney
SUSE Labs




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