linux-kernel - [PATCH 4.16 179/196] powerpc/lib: Fix off-by-one in alternate feature patching

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Date:   Sun, 22 Apr 2018 15:53:19 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Michael Ellerman <mpe@...erman.id.au>
Subject: [PATCH 4.16 179/196] powerpc/lib: Fix off-by-one in alternate feature patching

4.16-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Michael Ellerman <mpe@...erman.id.au>

commit b8858581febb050688e276b956796bc4a78299ed upstream.

When we patch an alternate feature section, we have to adjust any
relative branches that branch out of the alternate section.

But currently we have a bug if we have a branch that points to past
the last instruction of the alternate section, eg:

  FTR_SECTION_ELSE
  1:     b       2f
         or      6,6,6
  2:
  ALT_FTR_SECTION_END(...)
         nop

This will result in a relative branch at 1 with a target that equals
the end of the alternate section.

That branch does not need adjusting when it's moved to the non-else
location. Currently we do adjust it, resulting in a branch that goes
off into the link-time location of the else section, which is junk.

The fix is to not patch branches that have a target == end of the
alternate section.

Fixes: d20fe50a7b3c ("KVM: PPC: Book3S HV: Branch inside feature section")
Fixes: 9b1a735de64c ("powerpc: Add logic to patch alternative feature sections")
Cc: stable@...r.kernel.org # v2.6.27+
Signed-off-by: Michael Ellerman <mpe@...erman.id.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 arch/powerpc/lib/feature-fixups.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/arch/powerpc/lib/feature-fixups.c
+++ b/arch/powerpc/lib/feature-fixups.c
@@ -55,7 +55,7 @@ static int patch_alt_instruction(unsigne
 		unsigned int *target = (unsigned int *)branch_target(src);

 		/* Branch within the section doesn't need translating */
-		if (target < alt_start || target >= alt_end) {
+		if (target < alt_start || target > alt_end) {
 			instr = translate_branch(dest, src);
 			if (!instr)
 				return 1;