[PATCH] selinux: fix context string corruption in convert_context()

[PATCH] selinux: fix context string corruption in convert_context()

[Ondrej Mosnacek]

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

Reported-by: Milos Malik <mmalik@redhat.com>
Fixes: ee1a84fdfeed ("selinux: overhaul sidtab to fix bug and improve performance")
Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
---
 security/selinux/ss/services.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/security/selinux/ss/services.c b/security/selinux/ss/services.c
index 3a29e7c24ba9..a5813c7629c1 100644
--- a/security/selinux/ss/services.c
+++ b/security/selinux/ss/services.c
@@ -1946,7 +1946,14 @@ 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;
-- 
2.21.0

Re: [PATCH] selinux: fix context string corruption in convert_context()

[Stephen Smalley]

On 10/3/19 9:59 AM, Ondrej Mosnacek wrote:
> 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
> 
> Reported-by: Milos Malik <mmalik@redhat.com>
> Fixes: ee1a84fdfeed ("selinux: overhaul sidtab to fix bug and improve performance")
> Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>

Acked-by: Stephen Smalley <sds@tycho.nsa.gov>

> ---
>   security/selinux/ss/services.c | 9 ++++++++-
>   1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/security/selinux/ss/services.c b/security/selinux/ss/services.c
> index 3a29e7c24ba9..a5813c7629c1 100644
> --- a/security/selinux/ss/services.c
> +++ b/security/selinux/ss/services.c
> @@ -1946,7 +1946,14 @@ 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;
> 

Re: [PATCH] selinux: fix context string corruption in convert_context()

[Paul Moore]

On Thu, Oct 3, 2019 at 9:59 AM Ondrej Mosnacek <omosnace@redhat.com> wrote:
>
> 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
>
> Reported-by: Milos Malik <mmalik@redhat.com>
> Fixes: ee1a84fdfeed ("selinux: overhaul sidtab to fix bug and improve performance")
> Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
> ---
>  security/selinux/ss/services.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)

Thanks for finding and fixing this.  This looks like a good candidate
for stable so I went ahead and merged it into selinux/stable-5.4; if
any one has any objections to that, let me know by the end of the
week.

-- 
paul moore
www.paul-moore.com