Stack overflow in ecryptfs with /proc/pid/environ could lead to root (CVE-2016-1583)

This document (7017678) is provided subject to the disclaimer at the end of this document.

Environment

SUSE Linux Enterprise Server 12 (SLES 12)
SUSE Linux Enterprise Server 12 Service Pack 1 (SLES 12 SP1)

SUSE Linux Enterprise Server 11 Service Pack 4 (SLES 11 SP4)
SUSE Linux Enterprise Server 11 Service Pack 3 LTSS (SLES 11 SP3 LTSS)

Expanded Support 5 (RES5)
Expanded Support 6 (RES6)

Situation

Stacking filesystems, including ecryptfs, protect themselves against deep nesting, which would lead to kernel stack overflow, by tracking the recursion depth of filesystems. E.g. in ecryptfs, this is implemented in ecryptfs_mount() as follows:

s->s_stack_depth = path.dentry->d_sb->s_stack_depth + 1;

rc = -EINVAL;
if (s->s_stack_depth > FILESYSTEM_MAX_STACK_DEPTH)
{
pr_err("eCryptfs: maximum fs stacking depth exceeded\n");
goto out_free;
}

The files /proc/$pid/{mem,environ,cmdline}, when read, access the userspace memory of the target process, involving, if necessary, normal pagefault handling. If it was possible to mmap() them, an attacker could create a chain of e.g. /proc/$pid/environ mappings where process 1 has /proc/2/environ mapped into its environment area, process 2 has /proc/3/environ mapped into its environment area and so on. A read from /proc/1/environ would invoke the pagefault handler for process 1, which would invoke the pagefault handler for process 2 and so on. This would, again, lead to kernel stack overflow.

Resolution

SUSE has released patches to fix this vulnerability:

SLES 12
kernel 3.12.55-52.45.1 has been released 16th of June 2016

More patches (e.g. SLES 12 SP1 / SLES 11 SP4) are currently being reviewed by SUSE QA and will be released within the next days.

The following workaround is also available:

deinstall ecryptfs-utils (if not used)
Remove the setuid bit from /sbin/mount.ecryptfs_private by either a direct

chmod 755 /sbin/mount.ecryptfs_private
or to avoid the permissions framework undoing the change, temporary appending the line:

/sbin/mount.ecryptfs_private root:root 0755
to /etc/permissions.local and then running "chkstat --system"

Cause

Additional Information

Please note that this is a local exploit only.

CVE-2016-1583

Disclaimer

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