selinux: fix context string corruption in convert_context()

string_to_context_struct() may garble the context string, so we need to
copy back the contents again from the old context struct to avoid
storing the corrupted context.

Since string_to_context_struct() tokenizes (and therefore truncates) the
context string and we are later potentially copying it with kstrdup(),
this may eventually cause pieces of uninitialized kernel memory to be
disclosed to userspace (when copying to userspace based on the stored
length and not the null character).

How to reproduce on Fedora and similar:
    # dnf install -y memcached
    # systemctl start memcached
    # semodule -d memcached
    # load_policy
    # load_policy
    # systemctl stop memcached
    # ausearch -m AVC
    type=AVC msg=audit(1570090572.648:313): avc:  denied  { signal } for  pid=1 comm="systemd" scontext=system_u:system_r:init_t:s0 tcontext=system_u:object_r:unlabeled_t:s0 tclass=process permissive=0 trawcon=73797374656D5F75007400000000000070BE6E847296FFFF726F6D000096FFFF76

Cc: [email protected]
Reported-by: Milos Malik <[email protected]>
Fixes: ee1a84fdfeed ("selinux: overhaul sidtab to fix bug and improve performance")
Signed-off-by: Ondrej Mosnacek <[email protected]>
Acked-by: Stephen Smalley <[email protected]>
Signed-off-by: Paul Moore <[email protected]>

diff --git a/security/selinux/ss/services.c b/security/selinux/ss/services.c
index ca56cd045bf9e670e546bf346f00b7daf05560dc..39c317099d8d85c9b3f4ba8964d2c299d0b80aa1 100644 (file)
--- a/security/selinux/ss/services.c
+++ b/security/selinux/ss/services.c
@@ -1948,7 +1948,14 @@ static int convert_context(struct context *oldc, struct context *newc, void *p)
                rc = string_to_context_struct(args->newp, NULL, s,
                                              newc, SECSID_NULL);
                if (rc == -EINVAL) {
-                       /* Retain string representation for later mapping. */
+                       /*
+                        * Retain string representation for later mapping.
+                        *
+                        * IMPORTANT: We need to copy the contents of oldc->str
+                        * back into s again because string_to_context_struct()
+                        * may have garbled it.
+                        */
+                       memcpy(s, oldc->str, oldc->len);
                        context_init(newc);
                        newc->str = s;
                        newc->len = oldc->len;