Subject: Re: Kernel oops with 2.6.26, padlock and ipsec: probably problem with fpu state changes

From: Suresh Siddha <[email-masked]>
Date: Mon, 11 Aug 2008 12:01:32 -0700

On Sat, Aug 09, 2008 at 08:05:21PM -0700, Herbert Xu wrote:
> > void irq_ts_restore(int TS_state)
> > {
> >       if (!in_interrupt())
> >               return 0;
> 
> This check isn't necessary.
> 
> >
> >       if (TS_state)
> >               stts();
> > }
> 
> But yes this scheme looks good to me.

Appended the complete patch. Wolf, can you please help test this again
and check the perf aswell.

> > kernel_fpu_begin:
> >       ...
> >
> >       local_irq_disable();
> >
> >         if (me->status & TS_USEDFPU)
> >                 __save_init_fpu(me->task);
> >         else
> >                 clts();
> >
> >       local_irq_enable();
> >       ...
> 
> Couldn't we just move clts before the USEDFPU check? That huld
> close the window.

you are correct. as pre-emption is already disabled, we should be ok. But
given that we are taking another(clean) route to fix this issue, can leave the
current code as it is(and not do an unconditional clts()).
---

[patch] fix via padlock instruction usage with irq_ts_save/restore()

Wolfgang Walter reported this oops on his via C3 using padlock for
AES-encryption:

##################################################################

BUG: unable to handle kernel NULL pointer dereference at 000001f0
IP: [<c01028c5>] __switch_to+0x30/0x117
*pde = 00000000
Oops: 0002 [#1] PREEMPT
Modules linked in:

Pid: 2071, comm: sleep Not tainted (2.6.26 #11)
EIP: 0060:[<c01028c5>] EFLAGS: 00010002 CPU: 0
EIP is at __switch_to+0x30/0x117
EAX: 00000000 EBX: c0493300 ECX: dc48dd00 EDX: c0493300
ESI: dc48dd00 EDI: c0493530 EBP: c04cff8c ESP: c04cff7c
 DS: 007b ES: 007b FS: 0000 GS: 0033 SS: 0068
Process sleep (pid: 2071, ti=c04ce000 task=dc48dd00 task.ti=d2fe6000)
Stack: dc48df30 c0493300 00000000 00000000 d2fe7f44 c03b5b43 c04cffc8 00000046
       c0131856 0000005a dc472d3c c0493300 c0493470 d983ae00 00002696 00000000
       c0239f54 00000000 c04c4000 c04cffd8 c01025fe c04f3740 00049800 c04cffe0
Call Trace:
 [<c03b5b43>] ? schedule+0x285/0x2ff
 [<c0131856>] ? pm_qos_requirement+0x3c/0x53
 [<c0239f54>] ? acpi_processor_idle+0x0/0x434
 [<c01025fe>] ? cpu_idle+0x73/0x7f
 [<c03a4dcd>] ? rest_init+0x61/0x63
 =======================

Wolfgang also found out that adding kernel_fpu_begin() and kernel_fpu_end()
around the padlock instructions fix the oops.

Suresh wrote:

These padlock instructions though don't use/touch SSE registers, but it behaves
similar to other SSE instructions. For example, it might cause DNA faults
when cr0.ts is set. While this is a spurious DNA trap, it might cause
oops with the recent fpu code changes.

This is the code sequence  that is probably causing this problem:

a) new app is getting exec'd and it is somewhere in between
   start_thread() and flush_old_exec() in the load_xyz_binary()

b) At pont "a", task's fpu state (like TS_USEDFPU, used_math() etc) is
   cleared.

c) Now we get an interrupt/softirq which starts using these encrypt/decrypt
   routines in the network stack. This generates a math fault (as
   cr0.ts is '1') which sets TS_USEDFPU and restores the math that is
   in the task's xstate.

d) Return to exec code path, which does start_thread() which does
   free_thread_xstate() and sets xstate pointer to NULL while
   the TS_USEDFPU is still set.

e) At the next context switch from the new exec'd task to another task,
   we have a scenarios where TS_USEDFPU is set but xstate pointer is null.
   This can cause an oops during unlazy_fpu() in __switch_to()

Now:

1) This should happen with or with out pre-emption. Viro also encountered
   similar problem with out CONFIG_PREEMPT.

2) kernel_fpu_begin() and kernel_fpu_end() will fix this problem, because
   kernel_fpu_begin() will manually do a clts() and won't run in to the
   situation of setting TS_USEDFPU in step "c" above.

3) This was working before the fpu changes, because its a spurious
   math fault  which doesn't corrupt any fpu/sse registers and the task's
   math state was always in an allocated state.

With out the recent lazy fpu allocation changes, while we don't see oops,
there is a possible race still present in older kernels(for example,
while kernel is using kernel_fpu_begin() in some optimized clear/copy
page and an interrupt/softirq happens which uses these padlock
instructions generating DNA fault).

This is the failing scenario that existed even before the lazy fpu allocation
changes:

0. CPU's TS flag is set

1. kernel using FPU in some optimized copy  routine and while doing
kernel_fpu_begin() takes an interrupt just before doing clts()

2. Takes an interrupt and ipsec uses padlock instruction. And we
take a DNA fault as TS flag is still set.

3. We handle the DNA fault and set TS_USEDFPU and clear cr0.ts

4. We complete the padlock routine

5. Go back to step-1, which resumes clts() in kernel_fpu_begin(), finishes
the optimized copy routine and does kernel_fpu_end(). At this point,
we have cr0.ts again set to '1' but the task's TS_USEFPU is stilll
set and not cleared.

6. Now kernel resumes its user operation. And at the next context
switch, kernel sees it has do a FP save as TS_USEDFPU is still set
and then will do a unlazy_fpu() in __switch_to(). unlazy_fpu()
will take a DNA fault, as cr0.ts is '1' and now, because we are
in __switch_to(), math_state_restore() will get confused and will
restore the next task's FP state and will save it in prev tasks's FP state.
Remember, in __switch_to() we are already on the stack of the next task
but take a DNA fault for the prev task.

This causes the fpu leakage.

Fix the padlock instruction usage by calling them inside the
context of new routines irq_ts_save/restore(), which clear/restore cr0.ts
manually in the interrupt context. This will not generate spurious DNA
in the  context of the interrupt which will fix the oops encountered and
the possible FPU leakage issue.

Reported-and-bisected-by: Wolfgang Walter <[email-masked]>
Signed-off-by: Suresh Siddha <[email-masked]>
---

diff --git a/drivers/char/hw_random/via-rng.c b/drivers/char/hw_random/via-rng.c
index f7feae4..128202e 100644
--- a/drivers/char/hw_random/via-rng.c
+++ b/drivers/char/hw_random/via-rng.c
@@ -31,6 +31,7 @@
 #include <asm/io.h>
 #include <asm/msr.h>
 #include <asm/cpufeature.h>
+#include <asm/i387.h>

 #define PFX	KBUILD_MODNAME ": "
@@ -67,16 +68,23 @@ enum {
  * Another possible performance boost may come from simply buffering
  * until we have 4 bytes, thus returning a u32 at a time,
  * instead of the current u8-at-a-time.
+ *
+ * Padlock instructions can generate a spurious DNA fault, so
+ * we have to call them in the context of irq_ts_save/restore()
  */
 
 static inline u32 xstore(u32 *addr, u32 edx_in)
 {
 	u32 eax_out;
+	int ts_state;
+
+	ts_state = irq_ts_save();
 
 	asm(".byte 0x0F,0xA7,0xC0 /* xstore %%edi (addr=%0) */"
 		:"=m"(*addr), "=a"(eax_out)
 		:"D"(addr), "d"(edx_in));
 
+	irq_ts_restore(ts_state);
 	return eax_out;
 }
 
diff --git a/drivers/crypto/padlock-aes.c b/drivers/crypto/padlock-aes.c
index 54a2a16..bf2917d 100644
--- a/drivers/crypto/padlock-aes.c
+++ b/drivers/crypto/padlock-aes.c
@@ -16,6 +16,7 @@
 #include <linux/interrupt.h>
 #include <linux/kernel.h>
 #include <asm/byteorder.h>
+#include <asm/i387.h>
 #include "padlock.h"
 
 /* Control word. */
@@ -141,6 +142,12 @@ static inline void padlock_reset_key(void)
 	asm volatile ("pushfl; popfl");
 }
 
+/*
+ * While the padlock instructions don't use FP/SSE registers, they
+ * generate a spurious DNA fault when cr0.ts is '1'. These instructions
+ * should be used only inside the irq_ts_save/restore() context
+ */
+
 static inline void padlock_xcrypt(const u8 *input, u8 *output, void *key,
 				  void *control_word)
 {
@@ -205,15 +212,23 @@ static inline u8 *padlock_xcrypt_cbc(const u8 *input, u8 *output, void *key,
 static void aes_encrypt(struct crypto_tfm *tfm, u8 *out, const u8 *in)
 {
 	struct aes_ctx *ctx = aes_ctx(tfm);
+	int ts_state;
 	padlock_reset_key();
+
+	ts_state = irq_ts_save();
 	aes_crypt(in, out, ctx->E, &ctx->cword.encrypt);
+	irq_ts_restore(ts_state);
 }
 
 static void aes_decrypt(struct crypto_tfm *tfm, u8 *out, const u8 *in)
 {
 	struct aes_ctx *ctx = aes_ctx(tfm);
+	int ts_state;
 	padlock_reset_key();
+
+	ts_state = irq_ts_save();
 	aes_crypt(in, out, ctx->D, &ctx->cword.decrypt);
+	irq_ts_restore(ts_state);
 }
 
 static struct crypto_alg aes_alg = {
@@ -244,12 +259,14 @@ static int ecb_aes_encrypt(struct blkcipher_desc *desc,
 	struct aes_ctx *ctx = blk_aes_ctx(desc->tfm);
 	struct blkcipher_walk walk;
 	int err;
+	int ts_state;
 
 	padlock_reset_key();
 
 	blkcipher_walk_init(&walk, dst, src, nbytes);
 	err = blkcipher_walk_virt(desc, &walk);
 
+	ts_state = irq_ts_save();
 	while ((nbytes = walk.nbytes)) {
 		padlock_xcrypt_ecb(walk.src.virt.addr, walk.dst.virt.addr,
 				   ctx->E, &ctx->cword.encrypt,
@@ -257,6 +274,7 @@ static int ecb_aes_encrypt(struct blkcipher_desc *desc,
 		nbytes &= AES_BLOCK_SIZE - 1;
 		err = blkcipher_walk_done(desc, &walk, nbytes);
 	}
+	irq_ts_restore(ts_state);
 
 	return err;
 }
@@ -268,12 +286,14 @@ static int ecb_aes_decrypt(struct blkcipher_desc *desc,
 	struct aes_ctx *ctx = blk_aes_ctx(desc->tfm);
 	struct blkcipher_walk walk;
 	int err;
+	int ts_state;
 
 	padlock_reset_key();
 
 	blkcipher_walk_init(&walk, dst, src, nbytes);
 	err = blkcipher_walk_virt(desc, &walk);
 
+	ts_state = irq_ts_save();
 	while ((nbytes = walk.nbytes)) {
 		padlock_xcrypt_ecb(walk.src.virt.addr, walk.dst.virt.addr,
 				   ctx->D, &ctx->cword.decrypt,
@@ -281,7 +301,7 @@ static int ecb_aes_decrypt(struct blkcipher_desc *desc,
 		nbytes &= AES_BLOCK_SIZE - 1;
 		err = blkcipher_walk_done(desc, &walk, nbytes);
 	}
-
+	irq_ts_restore(ts_state);
 	return err;
 }
 
@@ -314,12 +334,14 @@ static int cbc_aes_encrypt(struct blkcipher_desc *desc,
 	struct aes_ctx *ctx = blk_aes_ctx(desc->tfm);
 	struct blkcipher_walk walk;
 	int err;
+	int ts_state;
 
 	padlock_reset_key();
 
 	blkcipher_walk_init(&walk, dst, src, nbytes);
 	err = blkcipher_walk_virt(desc, &walk);
 
+	ts_state = irq_ts_save();
 	while ((nbytes = walk.nbytes)) {
 		u8 *iv = padlock_xcrypt_cbc(walk.src.virt.addr,
 					    walk.dst.virt.addr, ctx->E,
@@ -329,6 +351,7 @@ static int cbc_aes_encrypt(struct blkcipher_desc *desc,
 		nbytes &= AES_BLOCK_SIZE - 1;
 		err = blkcipher_walk_done(desc, &walk, nbytes);
 	}
+	irq_ts_restore(ts_state);
 
 	return err;
 }
@@ -340,12 +363,14 @@ static int cbc_aes_decrypt(struct blkcipher_desc *desc,
 	struct aes_ctx *ctx = blk_aes_ctx(desc->tfm);
 	struct blkcipher_walk walk;
 	int err;
+	int ts_state;
 
 	padlock_reset_key();
 
 	blkcipher_walk_init(&walk, dst, src, nbytes);
 	err = blkcipher_walk_virt(desc, &walk);
 
+	ts_state = irq_ts_save();
 	while ((nbytes = walk.nbytes)) {
 		padlock_xcrypt_cbc(walk.src.virt.addr, walk.dst.virt.addr,
 				   ctx->D, walk.iv, &ctx->cword.decrypt,
@@ -354,6 +379,7 @@ static int cbc_aes_decrypt(struct blkcipher_desc *desc,
 		err = blkcipher_walk_done(desc, &walk, nbytes);
 	}
 
+	irq_ts_restore(ts_state);
 	return err;
 }
 
diff --git a/drivers/crypto/padlock-sha.c b/drivers/crypto/padlock-sha.c
index 40d5680..a7fbade 100644
--- a/drivers/crypto/padlock-sha.c
+++ b/drivers/crypto/padlock-sha.c
@@ -22,6 +22,7 @@
 #include <linux/interrupt.h>
 #include <linux/kernel.h>
 #include <linux/scatterlist.h>
+#include <asm/i387.h>
 #include "padlock.h"
 
 #define SHA1_DEFAULT_FALLBACK	"sha1-generic"
@@ -102,6 +103,7 @@ static void padlock_do_sha1(const char *in, char *out, int count)
 	 *     PadLock microcode needs it that big. */
 	char buf[128+16];
 	char *result = NEAREST_ALIGNED(buf);
+	int ts_state;
 
 	((uint32_t *)result)[0] = SHA1_H0;
 	((uint32_t *)result)[1] = SHA1_H1;
@@ -109,9 +111,12 @@ static void padlock_do_sha1(const char *in, char *out, int count)
 	((uint32_t *)result)[3] = SHA1_H3;
 	((uint32_t *)result)[4] = SHA1_H4;
  
+	/* prevent taking the spurious DNA fault with padlock. */
+	ts_state = irq_ts_save();
 	asm volatile (".byte 0xf3,0x0f,0xa6,0xc8" /* rep xsha1 */
 		      : "+S"(in), "+D"(result)
 		      : "c"(count), "a"(0));
+	irq_ts_restore(ts_state);
 
 	padlock_output_block((uint32_t *)result, (uint32_t *)out, 5);
 }
@@ -123,6 +128,7 @@ static void padlock_do_sha256(const char *in, char *out, int count)
 	 *     PadLock microcode needs it that big. */
 	char buf[128+16];
 	char *result = NEAREST_ALIGNED(buf);
+	int ts_state;
 
 	((uint32_t *)result)[0] = SHA256_H0;
 	((uint32_t *)result)[1] = SHA256_H1;
@@ -133,9 +139,12 @@ static void padlock_do_sha256(const char *in, char *out, int count)
 	((uint32_t *)result)[6] = SHA256_H6;
 	((uint32_t *)result)[7] = SHA256_H7;
 
+	/* prevent taking the spurious DNA fault with padlock. */
+	ts_state = irq_ts_save();
 	asm volatile (".byte 0xf3,0x0f,0xa6,0xd0" /* rep xsha256 */
 		      : "+S"(in), "+D"(result)
 		      : "c"(count), "a"(0));
+	irq_ts_restore(ts_state);
 
 	padlock_output_block((uint32_t *)result, (uint32_t *)out, 8);
 }
diff --git a/include/asm-x86/i387.h b/include/asm-x86/i387.h
index 96fa844..6d3b210 100644
--- a/include/asm-x86/i387.h
+++ b/include/asm-x86/i387.h
@@ -13,6 +13,7 @@
 #include <linux/sched.h>
 #include <linux/kernel_stat.h>
 #include <linux/regset.h>
+#include <linux/hardirq.h>
 #include <asm/asm.h>
 #include <asm/processor.h>
 #include <asm/sigcontext.h>
@@ -236,6 +237,37 @@ static inline void kernel_fpu_end(void)
 	preempt_enable();
 }
 
+/*
+ * Some instructions like VIA's padlock instructions generate a spurious
+ * DNA fault but don't modify SSE registers. And these instructions
+ * get used from interrupt context aswell. To prevent these kernel instructions
+ * in interrupt context interact wrongly with other user/kernel fpu usage, we
+ * should use them only in the context of irq_ts_save/restore()
+ */
+static inline int irq_ts_save(void)
+{
+	/*
+	 * If we are in process context, we are ok to take a spurious DNA fault.
+	 * Otherwise, doing clts() in process context require pre-emption to
+	 * be disabled or some heavy lifting like kernel_fpu_begin()
+	 */
+	if (!in_interrupt())
+		return 0;
+
+	if (read_cr0() & X86_CR0_TS) {
+		clts();
+		return 1;
+	}
+
+	return 0;
+}
+
+static inline void irq_ts_restore(int TS_state)
+{
+	if (TS_state)
+		stts();
+}
+
 #ifdef CONFIG_X86_64
 
 static inline void save_init_fpu(struct task_struct *tsk)