[PATCH 4.4 87/97] powerpc/lib: Fix off-by-one in alternate feature patching

From: Greg Kroah-Hartman
Date: Sun Apr 22 2018 - 10:19:12 EST

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


From: Michael Ellerman <mpe@xxxxxxxxxxxxxx>

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:

1: b 2f
or 6,6,6

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@xxxxxxxxxxxxxxx # v2.6.27+
Signed-off-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

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
@@ -53,7 +53,7 @@ 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;