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crypto: af_alg - avoid undefined behavior accessing salg_name

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Commit 3f69cc6076 ("crypto: af_alg - Allow arbitrarily long algorithm
names") made the kernel start accepting arbitrarily long algorithm names
in sockaddr_alg.  However, the actual length of the salg_name field
stayed at the original 64 bytes.

This is broken because the kernel can access indices >= 64 in salg_name,
which is undefined behavior -- even though the memory that is accessed
is still located within the sockaddr structure.  It would only be
defined behavior if the array were properly marked as arbitrary-length
(either by making it a flexible array, which is the recommended way
these days, or by making it an array of length 0 or 1).

We can't simply change salg_name into a flexible array, since that would
break source compatibility with userspace programs that embed
sockaddr_alg into another struct, or (more commonly) declare a
sockaddr_alg like 'struct sockaddr_alg sa = { .salg_name = "foo" };'.

One solution would be to change salg_name into a flexible array only
when '#ifdef __KERNEL__'.  However, that would keep userspace without an
easy way to actually use the longer algorithm names.

Instead, add a new structure 'sockaddr_alg_new' that has the flexible
array field, and expose it to both userspace and the kernel.
Make the kernel use it correctly in alg_bind().

This addresses the syzbot report
"UBSAN: array-index-out-of-bounds in alg_bind"
(https://syzkaller.appspot.com/bug?extid=92ead4eb8e26a26d465e).

Reported-by: syzbot+92ead4eb8e26a26d465e@syzkaller.appspotmail.com
Fixes: 3f69cc6076 ("crypto: af_alg - Allow arbitrarily long algorithm names")
Cc: <stable@vger.kernel.org> # v4.12+
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
This commit is contained in:
Eric Biggers
2020-10-26 13:07:15 -07:00
committed by Herbert Xu
parent 2d653936eb
commit 92eb6c3060
2 changed files with 23 additions and 3 deletions
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16
include/uapi/linux/if_alg.h
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@@ -24,6 +24,22 @@ struct sockaddr_alg {
__u8 salg_name[64];
};
/*
* Linux v4.12 and later removed the 64-byte limit on salg_name[]; it's now an
* arbitrary-length field. We had to keep the original struct above for source
* compatibility with existing userspace programs, though. Use the new struct
* below if support for very long algorithm names is needed. To do this,
* allocate 'sizeof(struct sockaddr_alg_new) + strlen(algname) + 1' bytes, and
* copy algname (including the null terminator) into salg_name.
*/
struct sockaddr_alg_new {
__u16 salg_family;
__u8 salg_type[14];
__u32 salg_feat;
__u32 salg_mask;
__u8 salg_name[];
};
struct af_alg_iv {
__u32 ivlen;
__u8 iv[0];