Re: [PATCH] edac: fix buffer overrun if no suitable bandwidth found

Andrew Morton
2012-10-23T21:09:43Z

On Tue, 23 Oct 2012 22:49:26 +0200
Borislav Petkov <bp@amd64.org> wrote:

> Now you only need to give me a proper patch with your S-O-B and we're
> ready to go :).

who, me, what?!?! Sounds stressful.

umm, OK here we go.

What I don't understand is the effects of the bug. If the present code
can indeed start using the n+1th element of the array then it's writing
random garbage into the hardware. If this can ever happen then I
suspect that yes, a cc:stable is needed?

From: Andrew Morton <akpm@linux-foundation.org>
Subject: drivers/edac/amd64_edac.c:__amd64_set_scrub_rate(): avoid overindexing scrubrates[]

If none of the elements in scrubrates[] matches, this loop will cause
__amd64_set_scrub_rate() to incorrectly use the n+1th element.

As the function is designed to use the final scrubrates[] element in the
case of no match, we can fix this bug by simply terminating the array
search at the n-1th element.

Reported-by: Denis Kirjanov <kirjanov@gmail.com>
Cc: Doug Thompson <dougthompson@xmission.com>
Cc: Borislav Petkov <borislav.petkov@amd.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

drivers/edac/amd64_edac.c | 11 ++++-------
1 file changed, 4 insertions(+), 7 deletions(-)

diff -puN drivers/edac/amd64_edac.c~drivers-edac-amd64_edacc-__amd64_set_scrub_rate-avoid-overindexing-scrubrates drivers/edac/amd64_edac.c
--- a/drivers/edac/amd64_edac.c~drivers-edac-amd64_edacc-__amd64_set_scrub_rate-avoid-overindexing-scrubrates
+++ a/drivers/edac/amd64_edac.c
@@ -170,8 +170,11 @@ static int __amd64_set_scrub_rate(struct
* memory controller and apply to register. Search for the first
* bandwidth entry that is greater or equal than the setting requested
* and program that. If at last entry, turn off DRAM scrubbing.
+ *
+ * If no suitable bandwidth is found, turn off DRAM scrubbing entirely
+ * by falling back to the last element in scrubrates[].
*/
- for (i = 0; i < ARRAY_SIZE(scrubrates); i++) {
+ for (i = 0; i < ARRAY_SIZE(scrubrates) - 1; i++) {
/*
* skip scrub rates which aren't recommended
* (see F10 BKDG, F3x58)
@@ -181,12 +184,6 @@ static int __amd64_set_scrub_rate(struct

if (scrubrates[i].bandwidth <= new_bw)
break;
-
- /*
- * if no suitable bandwidth found, turn off DRAM scrubbing
- * entirely by falling back to the last element in the
- * scrubrates array.
- */
}

scrubval = scrubrates[i].scrubval;
_