On 09/18/2016 11:05 AM, Jann Horn wrote:
> This adds a new ptrace_may_access_noncurrent() method that
> uses the supplied credentials instead of those of the
> current task. (However, the current task may still be
> inspected for auditing purposes, e.g. by the Smack LSM.)
>
> procfs used the caller's creds for a few ptrace_may_access()
> checks at read() time, which made a confused deputy attack
> by passing an FD to a procfs file to a setuid program
> possible. Therefore, the following was a local userspace
> ASLR bypass:
>
> rm -f /tmp/foobar
> procmail <(echo 'DEFAULT=/tmp/foobar') </proc/1/stat
> (
> echo 'obase=16'
> cut -d' ' -f26-30,45-51 </tmp/foobar | tr ' ' '\n'
> ) | bc
> rm /tmp/foobar
>
> procmail is installed setuid root on Debian and read()s
> data from stdin before dropping privs, so the
> ptrace_may_access() check in the VFS read handler of
> /proc/1/stat passes. Procmail then dumps the read data
> to a user-accessible file (/tmp/foobar here).
>
> Signed-off-by: Jann Horn <jann@xxxxxxxxx>
> ---
> fs/proc/array.c | 3 +-
> fs/proc/base.c | 63 ++++++++++++++++++++++++++++++++++-------
> fs/proc/internal.h | 14 +++++++++
> include/linux/lsm_hooks.h | 3 +-
> include/linux/ptrace.h | 5 ++++
> include/linux/security.h | 10 ++++---
> kernel/ptrace.c | 40 +++++++++++++++++++-------
> security/apparmor/include/ipc.h | 2 +-
> security/apparmor/ipc.c | 4 +--
> security/apparmor/lsm.c | 14 +++++++--
> security/commoncap.c | 8 +++---
> security/security.c | 5 ++--
> security/selinux/hooks.c | 4 +--
> security/smack/smack_lsm.c | 18 ++++++++----
> security/yama/yama_lsm.c | 9 ++++--
> 15 files changed, 150 insertions(+), 52 deletions(-)
>
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index 13185a6..f9a0be7 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -2133,10 +2133,10 @@ static int selinux_binder_transfer_file(struct task_struct *from,
> }
>
> static int selinux_ptrace_access_check(struct task_struct *child,
> - unsigned int mode)
> + unsigned int mode, const struct cred *cred)
> {
> if (mode & PTRACE_MODE_READ) {
> - u32 sid = current_sid();
> + u32 sid = cred_sid(cred);
> u32 csid = task_sid(child);
> return avc_has_perm(sid, csid, SECCLASS_FILE, FILE__READ, NULL);
> }
For consistency, don't you also need to change the next line of code to
use cred_has_perm() rather than current_has_perm()?
--
To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
