Patchwork [V3,1/2] evm: Don't deadlock if a crypto algorithm is unavailable

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Submitter Matthew Garrett
Date June 8, 2018, 9:57 p.m.
Message ID <20180608215743.224253-2-mjg59@google.com>
Download mbox | patch
Permalink /patch/10455275/
State Not Applicable
Delegated to: Herbert Xu
Headers show

Comments

Matthew Garrett - June 8, 2018, 9:57 p.m.
When EVM attempts to appraise a file signed with a crypto algorithm the
kernel doesn't have support for, it will cause the kernel to trigger a
module load. If the EVM policy includes appraisal of kernel modules this
will in turn call back into EVM - since EVM is holding a lock until the
crypto initialisation is complete, this triggers a deadlock. Add a
CRYPTO_NOLOAD flag and skip module loading if it's set, and add that flag
in the EVM case in order to fail gracefully with an error message
instead of deadlocking.

Signed-off-by: Matthew Garrett <mjg59@google.com>
---
 crypto/api.c                        | 2 +-
 include/linux/crypto.h              | 5 +++++
 security/integrity/evm/evm_crypto.c | 3 ++-
 3 files changed, 8 insertions(+), 2 deletions(-)
Matthew Garrett - June 12, 2018, 5:43 p.m.
On Fri, Jun 8, 2018 at 2:57 PM Matthew Garrett <mjg59@google.com> wrote:
>
> When EVM attempts to appraise a file signed with a crypto algorithm the
> kernel doesn't have support for, it will cause the kernel to trigger a
> module load. If the EVM policy includes appraisal of kernel modules this
> will in turn call back into EVM - since EVM is holding a lock until the
> crypto initialisation is complete, this triggers a deadlock. Add a
> CRYPTO_NOLOAD flag and skip module loading if it's set, and add that flag
> in the EVM case in order to fail gracefully with an error message
> instead of deadlocking.

Hi Herbert,

Does this explain the problem sufficiently?
Herbert Xu - June 13, 2018, 6:33 a.m.
On Fri, Jun 08, 2018 at 02:57:42PM -0700, Matthew Garrett wrote:
> When EVM attempts to appraise a file signed with a crypto algorithm the
> kernel doesn't have support for, it will cause the kernel to trigger a
> module load. If the EVM policy includes appraisal of kernel modules this
> will in turn call back into EVM - since EVM is holding a lock until the
> crypto initialisation is complete, this triggers a deadlock. Add a
> CRYPTO_NOLOAD flag and skip module loading if it's set, and add that flag
> in the EVM case in order to fail gracefully with an error message
> instead of deadlocking.
> 
> Signed-off-by: Matthew Garrett <mjg59@google.com>

Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Mimi Zohar - June 13, 2018, 1:50 p.m.
On Wed, 2018-06-13 at 14:33 +0800, Herbert Xu wrote:
> On Fri, Jun 08, 2018 at 02:57:42PM -0700, Matthew Garrett wrote:
> > When EVM attempts to appraise a file signed with a crypto algorithm the
> > kernel doesn't have support for, it will cause the kernel to trigger a
> > module load. If the EVM policy includes appraisal of kernel modules this
> > will in turn call back into EVM - since EVM is holding a lock until the
> > crypto initialisation is complete, this triggers a deadlock. Add a
> > CRYPTO_NOLOAD flag and skip module loading if it's set, and add that flag
> > in the EVM case in order to fail gracefully with an error message
> > instead of deadlocking.
> > 
> > Signed-off-by: Matthew Garrett <mjg59@google.com>
> 
> Acked-by: Herbert Xu <herbert@gondor.apana.org.au>

Thanks!  This patch and "evm: Allow non-SHA1 digital signatures" are
now queued in the next-integrity-queued branch.

Mimi

Patch

diff --git a/crypto/api.c b/crypto/api.c
index 0ee632bba064..7aca9f86c5f3 100644
--- a/crypto/api.c
+++ b/crypto/api.c
@@ -229,7 +229,7 @@  static struct crypto_alg *crypto_larval_lookup(const char *name, u32 type,
 	mask &= ~(CRYPTO_ALG_LARVAL | CRYPTO_ALG_DEAD);
 
 	alg = crypto_alg_lookup(name, type, mask);
-	if (!alg) {
+	if (!alg && !(mask & CRYPTO_NOLOAD)) {
 		request_module("crypto-%s", name);
 
 		if (!((type ^ CRYPTO_ALG_NEED_FALLBACK) & mask &
diff --git a/include/linux/crypto.h b/include/linux/crypto.h
index 6eb06101089f..e8839d3a7559 100644
--- a/include/linux/crypto.h
+++ b/include/linux/crypto.h
@@ -112,6 +112,11 @@ 
  */
 #define CRYPTO_ALG_OPTIONAL_KEY		0x00004000
 
+/*
+ * Don't trigger module loading
+ */
+#define CRYPTO_NOLOAD			0x00008000
+
 /*
  * Transform masks and values (for crt_flags).
  */
diff --git a/security/integrity/evm/evm_crypto.c b/security/integrity/evm/evm_crypto.c
index 494da5fcc092..bea847dc0919 100644
--- a/security/integrity/evm/evm_crypto.c
+++ b/security/integrity/evm/evm_crypto.c
@@ -97,7 +97,8 @@  static struct shash_desc *init_desc(char type)
 		mutex_lock(&mutex);
 		if (*tfm)
 			goto out;
-		*tfm = crypto_alloc_shash(algo, 0, CRYPTO_ALG_ASYNC);
+		*tfm = crypto_alloc_shash(algo, 0,
+					  CRYPTO_ALG_ASYNC | CRYPTO_NOLOAD);
 		if (IS_ERR(*tfm)) {
 			rc = PTR_ERR(*tfm);
 			pr_err("Can not allocate %s (reason: %ld)\n", algo, rc);