On February 14, 2024 7:45:43 PM Kees Cook [off-list ref] wrote:

On Wed, Feb 14, 2024 at 08:53:52AM -0800, Casey Schaufler wrote:

quoted

On 2/14/2024 8:05 AM, Jann Horn wrote:

quoted

security_setselfattr() has an integer overflow bug that leads to
out-of-bounds access when userspace provides bogus input:
`lctx->ctx_len + sizeof(*lctx)` is checked against `lctx->len` (and,
redundantly, also against `size`), but there are no checks on
`lctx->ctx_len`.
Therefore, userspace can provide an `lsm_ctx` with `->ctx_len` set to a
value between `-sizeof(struct lsm_ctx)` and -1, and this bogus `->ctx_len`
will then be passed to an LSM module as a buffer length, causing LSM
modules to perform out-of-bounds accesses.

The following reproducer will demonstrate this under ASAN (if AppArmor is
loaded as an LSM):

#define _GNU_SOURCE
#include <unistd.h>
#include <stdint.h>
#include <stdlib.h>
#include <sys/syscall.h>

struct lsm_ctx {
uint64_t id;
uint64_t flags;
uint64_t len;
uint64_t ctx_len;

Do we want to take the opportunity to reduce this to u32 for len and u32
for ctx_len? All FS operations are limited to INT_MAX anyway...

Not at this point, no.

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paul-moore.com