Patch "crypto: Work around deallocated stack frame reference gcc bug on sparc." has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

crypto: Work around deallocated stack frame reference gcc bug on sparc.

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
crypto-work-around-deallocated-stack-frame-reference-gcc-bug-on-sparc.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: David Miller <[email protected]>
Date: Fri, 2 Jun 2017 11:28:54 -0400
Subject: crypto: Work around deallocated stack frame reference gcc bug on sparc.

From: David Miller <[email protected]>

commit d41519a69b35b10af7fda867fb9100df24fdf403 upstream.

On sparc, if we have an alloca() like situation, as is the case with SHASH_DESC_ON_STACK(), we can end up referencing deallocated stack memory. The result can be that the value is clobbered if a trap or interrupt arrives at just the right instruction.

It only occurs if the function ends returning a value from that alloca() area and that value can be placed into the return value register using a single instruction.

For example, in lib/libcrc32c.c:crc32c() we end up with a return sequence like:

        return  %i7+8
         lduw   [%o5+16], %o0   ! MEM[(u32 *)__shash_desc.1_10 + 16B],

%o5 holds the base of the on-stack area allocated for the shash descriptor. But the return released the stack frame and the register window.

So if an intererupt arrives between 'return' and 'lduw', then the value read at %o5+16 can be corrupted.

Add a data compiler barrier to work around this problem. This is exactly what the gcc fix will end up doing as well, and it absolutely should not change the code generated for other cpus (unless gcc on them has the same bug :-)

With crucial insight from Eric Sandeen.

---
 fs/btrfs/hash.c |    5 ++++-
 fs/f2fs/f2fs.h  |    5 ++++-
 lib/libcrc32c.c |    6 ++++--
 3 files changed, 12 insertions(+), 4 deletions(-)

@@ -38,6 +38,7 @@ u32 btrfs_crc32c(u32 crc, const void *ad
 {
 	SHASH_DESC_ON_STACK(shash, tfm);
 	u32 *ctx = (u32 *)shash_desc_ctx(shash);
+	u32 retval;
 	int err;
 
 	shash->tfm = tfm;

@@ -47,5 +48,7 @@ u32 btrfs_crc32c(u32 crc, const void *ad
 	err = crypto_shash_update(shash, address, length);
 	BUG_ON(err);
 
-	return *ctx;
+	retval = *ctx;
+	barrier_data(ctx);
+	return retval;
 }

@@ -948,6 +948,7 @@ static inline u32 f2fs_crc32(struct f2fs
 {
 	SHASH_DESC_ON_STACK(shash, sbi->s_chksum_driver);
 	u32 *ctx = (u32 *)shash_desc_ctx(shash);
+	u32 retval;
 	int err;
 
 	shash->tfm = sbi->s_chksum_driver;

@@ -957,7 +958,9 @@ static inline u32 f2fs_crc32(struct f2fs
 	err = crypto_shash_update(shash, address, length);
 	BUG_ON(err);
 
-	return *ctx;
+	retval = *ctx;
+	barrier_data(ctx);
+	return retval;
 }
 
 static inline bool f2fs_crc_valid(struct f2fs_sb_info *sbi, __u32 blk_crc,

@@ -43,7 +43,7 @@ static struct crypto_shash *tfm;
 u32 crc32c(u32 crc, const void *address, unsigned int length)
 {
 	SHASH_DESC_ON_STACK(shash, tfm);
-	u32 *ctx = (u32 *)shash_desc_ctx(shash);
+	u32 ret, *ctx = (u32 *)shash_desc_ctx(shash);
 	int err;
 
 	shash->tfm = tfm;

@@ -53,7 +53,9 @@ u32 crc32c(u32 crc, const void *address,
 	err = crypto_shash_update(shash, address, length);
 	BUG_ON(err);
 
-	return *ctx;
+	ret = *ctx;
+	barrier_data(ctx);
+	return ret;
 }
 
 EXPORT_SYMBOL(crc32c);

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/ila_xlat-add-missing-hash-secret-initialization.patch queue-4.9/crypto-work-around-deallocated-stack-frame-reference-gcc-bug-on-sparc.patch