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Subject[PATCH] reiserfs: fix broken xattr handling (heap corruption, bad retval)
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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@vger.kernel.org
Signed-off-by: Jann Horn <jannh@google.com>
---
Triggering the bug:

root@debian:/home/user# mount -o user_xattr reiserimg reisermount/
root@debian:/home/user# cd reisermount/
root@debian:/home/user/reisermount# touch test_file
root@debian:/home/user/reisermount# setfattr -n user.foo1 -v A test_file
root@debian:/home/user/reisermount# setfattr -n user.foo2 -v A test_file
root@debian:/home/user/reisermount# setfattr -n user.foo3 -v A test_file
root@debian:/home/user/reisermount# setfattr -n user.foo4 -v A test_file
root@debian:/home/user/reisermount# setfattr -n user.foo5 -v A test_file
root@debian:/home/user/reisermount# setfattr -n user.foo6 -v A test_file
root@debian:/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@debian:/home/user/reisermount# gcc -o xattr_test xattr_test.c
root@debian:/home/user/reisermount# ./xattr_test test_file
Segmentation fault
root@debian:/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;
}
--
2.18.0.597.ga71716f1ad-goog
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