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

Wolfgang Walter
2008-08-09T01:28:57Z

On Saturday 09 August 2008, Suresh Siddha wrote:
> Walter, Viro,
>
> As I can't test, can you please test this and Ack.
>
> thanks,
> suresh

* AES-CBC

Seems to work here: running since 73 minutes my test which usually crashes the
machine after 1-2 minutes :-).

Enclosing the whole encryption/decryption is a good idea, by the way. With my
modification I observed a large latency (up to 500ms) for some packets over
the esp-tunnel (every 5 to 10 seconds there were some of them). This is not
the case with your patch.

I did some simple performance tests. As far as I can see throughput and
latency (i.e. routing packets as ipsec-gateway) are at least as good as with
2.6.25.13.

* RNG

did several time

dd if=/dev/hwrng bs=1024 count=$((1024*100)) of=/dev/zero

Works fine. Observed no performance degradation compared to 2.6.25.

* Hash-Engine

Can't test that as I don't have an VIA Ester or newer.

I'm very glad this issue is solved. Thank you and Herbert for your help.

> ---
> [patch] fix via padlock instruction usage with kernel_fpu_begin/end()
>
> 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 dynamic 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).
>
> For now, fix the padlock instruction usage by calling them inside the
> context of kernel_fpu_begin() and kernel_fpu_end()
>
> Next steps:
>
> a) Based on the need, possible introduction of light weight kernel_fpu_*
> routines which will optimize the padlock usage case, where they don't
> touch SSE/FPU registers, but generate DNA.
>
> b) Looking deeper, do we need to disable interrupts in the
kernel_fpu_begin()?
> Is there a recursive case, where interrupt context also touches FPU/SSE
> registers?
>
> Reported-and-bisected-by: Wolfgang Walter <wolfgang.walter@stwm.de>
> Signed-off-by: Wolfgang Walter <wolfgang.walter@stwm.de>
> Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
> ---
>
> diff --git a/drivers/char/hw_random/via-rng.c
b/drivers/char/hw_random/via-rng.c
> index f7feae4..3dee9e5 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 +68,22 @@ 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 will call them in the context of kernel_fpu_[begin,end].
> */
>
> static inline u32 xstore(u32 *addr, u32 edx_in)
> {
> u32 eax_out;
>
> + kernel_fpu_begin();
> +
> asm(".byte 0x0F,0xA7,0xC0 /* xstore %%edi (addr=%0) */"
> :"=m"(*addr), "=a"(eax_out)
> :"D"(addr), "d"(edx_in));
>
> + kernel_fpu_end();
> return eax_out;
> }
>
> diff --git a/drivers/crypto/padlock-aes.c b/drivers/crypto/padlock-aes.c
> index 54a2a16..2c96d85 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 kernel_fpu_[begin, end] context.
> + */
> +
> static inline void padlock_xcrypt(const u8 *input, u8 *output, void *key,
> void *control_word)
> {
> @@ -206,14 +213,20 @@ static void aes_encrypt(struct crypto_tfm *tfm, u8
*out, const u8 *in)
> {
> struct aes_ctx *ctx = aes_ctx(tfm);
> padlock_reset_key();
> +
> + kernel_fpu_begin();
> aes_crypt(in, out, ctx->E, &ctx->cword.encrypt);
> + kernel_fpu_end();
> }
>
> static void aes_decrypt(struct crypto_tfm *tfm, u8 *out, const u8 *in)
> {
> struct aes_ctx *ctx = aes_ctx(tfm);
> padlock_reset_key();
> +
> + kernel_fpu_begin();
> aes_crypt(in, out, ctx->D, &ctx->cword.decrypt);
> + kernel_fpu_end();
> }
>
> static struct crypto_alg aes_alg = {
> @@ -250,6 +263,7 @@ static int ecb_aes_encrypt(struct blkcipher_desc *desc,
> blkcipher_walk_init(&walk, dst, src, nbytes);
> err = blkcipher_walk_virt(desc, &walk);
>
> + kernel_fpu_begin();
> while ((nbytes = walk.nbytes)) {
> padlock_xcrypt_ecb(walk.src.virt.addr, walk.dst.virt.addr,
> ctx->E, &ctx->cword.encrypt,
> @@ -257,6 +271,7 @@ static int ecb_aes_encrypt(struct blkcipher_desc *desc,
> nbytes &= AES_BLOCK_SIZE - 1;
> err = blkcipher_walk_done(desc, &walk, nbytes);
> }
> + kernel_fpu_end();
>
> return err;
> }
> @@ -274,6 +289,7 @@ static int ecb_aes_decrypt(struct blkcipher_desc *desc,
> blkcipher_walk_init(&walk, dst, src, nbytes);
> err = blkcipher_walk_virt(desc, &walk);
>
> + kernel_fpu_begin();
> while ((nbytes = walk.nbytes)) {
> padlock_xcrypt_ecb(walk.src.virt.addr, walk.dst.virt.addr,
> ctx->D, &ctx->cword.decrypt,
> @@ -281,6 +297,7 @@ static int ecb_aes_decrypt(struct blkcipher_desc *desc,
> nbytes &= AES_BLOCK_SIZE - 1;
> err = blkcipher_walk_done(desc, &walk, nbytes);
> }
> + kernel_fpu_end();
>
> return err;
> }
> @@ -320,6 +337,7 @@ static int cbc_aes_encrypt(struct blkcipher_desc *desc,
> blkcipher_walk_init(&walk, dst, src, nbytes);
> err = blkcipher_walk_virt(desc, &walk);
>
> + kernel_fpu_begin();
> while ((nbytes = walk.nbytes)) {
> u8 *iv = padlock_xcrypt_cbc(walk.src.virt.addr,
> walk.dst.virt.addr, ctx->E,
> @@ -329,6 +347,7 @@ static int cbc_aes_encrypt(struct blkcipher_desc *desc,
> nbytes &= AES_BLOCK_SIZE - 1;
> err = blkcipher_walk_done(desc, &walk, nbytes);
> }
> + kernel_fpu_end();
>
> return err;
> }
> @@ -346,6 +365,7 @@ static int cbc_aes_decrypt(struct blkcipher_desc *desc,
> blkcipher_walk_init(&walk, dst, src, nbytes);
> err = blkcipher_walk_virt(desc, &walk);
>
> + kernel_fpu_begin();
> while ((nbytes = walk.nbytes)) {
> padlock_xcrypt_cbc(walk.src.virt.addr, walk.dst.virt.addr,
> ctx->D, walk.iv, &ctx->cword.decrypt,
> @@ -353,6 +373,7 @@ static int cbc_aes_decrypt(struct blkcipher_desc *desc,
> nbytes &= AES_BLOCK_SIZE - 1;
> err = blkcipher_walk_done(desc, &walk, nbytes);
> }
> + kernel_fpu_end();
>
> return err;
> }
> diff --git a/drivers/crypto/padlock-sha.c b/drivers/crypto/padlock-sha.c
> index 40d5680..cea8830 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"
> @@ -109,9 +110,12 @@ 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. */
> + kernel_fpu_begin();
> asm volatile (".byte 0xf3,0x0f,0xa6,0xc8" /* rep xsha1 */
> : "+S"(in), "+D"(result)
> : "c"(count), "a"(0));
> + kernel_fpu_end();
>
> padlock_output_block((uint32_t *)result, (uint32_t *)out, 5);
> }
> @@ -133,9 +137,12 @@ 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. */
> + kernel_fpu_begin();
> asm volatile (".byte 0xf3,0x0f,0xa6,0xd0" /* rep xsha256 */
> : "+S"(in), "+D"(result)
> : "c"(count), "a"(0));
> + kernel_fpu_end();
>
> padlock_output_block((uint32_t *)result, (uint32_t *)out, 8);
> }
>
>

Regards,
--
Wolfgang Walter
Studentenwerk München
Anstalt des öffentlichen Rechts