Re: sha512: make it work, undo percpu message schedule

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Le vendredi 13 janvier 2012 à 18:08 +1100, Herbert Xu a écrit :
> On Fri, Jan 13, 2012 at 02:55:14AM +0300, Alexey Dobriyan wrote:
> >
> > Herbert, I couldn't come up with a single scenario. :-(
> > But the bug is easy to reproduce.
> 
> OK, does this patch work for you?
> 
> commit 31f4e55c09c1170f8b813c14b1299b70f50db414
> Author: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
> Date:   Fri Jan 13 18:06:50 2012 +1100
> 
>     crypto: sha512 - Fix msg_schedule race
>     
>     The percpu msg_schedule setup was unsafe as a user in a process
>     context can be interrupted by a softirq user which would then
>     scribble over the exact same work area.  This was discovered by
>     Steffen Klassert.
>     
>     This patch based on ideas from Eric Dumazet fixes this by using
>     two independent work areas.
>     
>     Reported-by: Alexey Dobriyan <adobriyan@xxxxxxxxx>
>     Signed-off-by: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
> 

I wonder ...

With 4096 cpus, do we really want to reserve 5242880 bytes of memory for
this function ?

What about following patch instead ?

(Trying a dynamic memory allocation, and fallback on a single
pre-allocated bloc of memory, shared by all cpus, protected by a
spinlock)

diff --git a/crypto/sha512_generic.c b/crypto/sha512_generic.c
index 9ed9f60..5c80a76 100644
--- a/crypto/sha512_generic.c
+++ b/crypto/sha512_generic.c
@@ -21,7 +21,6 @@
 #include <linux/percpu.h>
 #include <asm/byteorder.h>
 
-static DEFINE_PER_CPU(u64[80], msg_schedule);
 
 static inline u64 Ch(u64 x, u64 y, u64 z)
 {
@@ -87,10 +86,16 @@ static void
 sha512_transform(u64 *state, const u8 *input)
 {
 	u64 a, b, c, d, e, f, g, h, t1, t2;
-
+	static u64 msg_schedule[80];
+	static DEFINE_SPINLOCK(msg_schedule_lock);
 	int i;
-	u64 *W = get_cpu_var(msg_schedule);
+	u64 *W = kzalloc(sizeof(msg_schedule), GFP_ATOMIC | __GFP_NOWARN);
 
+	
+	if (!W) {
+		spin_lock_bh(&msg_schedule_lock);
+		W = msg_schedule;
+	}
 	/* load the input */
         for (i = 0; i < 16; i++)
                 LOAD_OP(i, W, input);
@@ -128,8 +133,11 @@ sha512_transform(u64 *state, const u8 *input)
 
 	/* erase our data */
 	a = b = c = d = e = f = g = h = t1 = t2 = 0;
-	memset(W, 0, sizeof(__get_cpu_var(msg_schedule)));
-	put_cpu_var(msg_schedule);
+	memset(W, 0, sizeof(msg_schedule));
+	if (W == msg_schedule)
+		spin_unlock_bh(&msg_schedule_lock);
+	else
+		kfree(W);
 }
 
 static int


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