[Trusty SRU][PATCH 2/2] aio: mark AIO pseudo-fs noexec

[Kleber Sacilotto de Souza]

From: Jann Horn <jann at thejh.net>

CVE-2016-10044

This ensures that do_mmap() won't implicitly make AIO memory mappings
executable if the READ_IMPLIES_EXEC personality flag is set.  Such
behavior is problematic because the security_mmap_file LSM hook doesn't
catch this case, potentially permitting an attacker to bypass a W^X
policy enforced by SELinux.

I have tested the patch on my machine.

To test the behavior, compile and run this:

    #define _GNU_SOURCE
    #include <unistd.h>
    #include <sys/personality.h>
    #include <linux/aio_abi.h>
    #include <err.h>
    #include <stdlib.h>
    #include <stdio.h>
    #include <sys/syscall.h>

    int main(void) {
        personality(READ_IMPLIES_EXEC);
        aio_context_t ctx = 0;
        if (syscall(__NR_io_setup, 1, &ctx))
            err(1, "io_setup");

        char cmd[1000];
        sprintf(cmd, "cat /proc/%d/maps | grep -F '/[aio]'",
            (int)getpid());
        system(cmd);
        return 0;
    }

In the output, "rw-s" is good, "rwxs" is bad.

Signed-off-by: Jann Horn <jann at thejh.net>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
(backported from commit 22f6b4d34fcf039c63a94e7670e0da24f8575a5a)
[kleber: replaced the inline magic number by the macro AIO_RING_MAGIC,
 as done by 8dc4379e17cddad7b2088a3f300ded50d2a6d493 mainline]
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza at canonical.com>
---
 fs/aio.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/fs/aio.c b/fs/aio.c
index db7adacb5543..8dd68431ff62 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -208,7 +208,12 @@ static struct dentry *aio_mount(struct file_system_type *fs_type,
 	static const struct dentry_operations ops = {
 		.d_dname	= simple_dname,
 	};
-	return mount_pseudo(fs_type, "aio:", NULL, &ops, 0xa10a10a1);
+	struct dentry *root = mount_pseudo(fs_type, "aio:", NULL, &ops,
+					   AIO_RING_MAGIC);
+
+	if (!IS_ERR(root))
+		root->d_sb->s_iflags |= SB_I_NOEXEC;
+	return root;
 }
 
 /* aio_setup
-- 
2.14.1