It turns out that doing the SELinux MAC checks for mmap() before the
DAC checks was causing users and the SELinux policy folks headaches
as users were seeing a lot of SELinux AVC denials for the
memprotect:mmap_zero permission that would have also been denied by
the normal DAC capability checks (CAP_SYS_RAWIO).
Example:
# cat mmap_test.c
#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
#include <sys/mman.h>
int main(int argc, char *argv[])
{
int rc;
void *mem;
mem = mmap(0x0, 4096,
PROT_READ | PROT_WRITE,
MAP_PRIVATE | MAP_ANONYMOUS | MAP_FIXED, -1, 0);
if (mem == MAP_FAILED)
return errno;
printf("mem = %p\n", mem);
munmap(mem, 4096);
return 0;
}
# gcc -g -O0 -o mmap_test mmap_test.c
# ./mmap_test
mem = (nil)
# ausearch -m AVC | grep mmap_zero
type=AVC msg=audit(...): avc: denied { mmap_zero }
for pid=1025 comm="mmap_test"
scontext=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023
tcontext=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023
tclass=memprotect
This patch corrects things so that when the above example is run by a
user without CAP_SYS_RAWIO the SELinux AVC is no longer generated as
the DAC capability check fails before the SELinux permission check.
Signed-off-by: Paul Moore <pmoore@redhat.com>
Acked-by: Stephen Smalley <sds@tycho.nsa.gov>
98883bfd9d