[PATCH 3.16 27/47] x86/ptrace: Fix possible spectre-v1 in ptrace_get_debugreg()

[Ben Hutchings]

3.16.76-rc1 review patch.  If anyone has any objections, please let me know.

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

From: Dianzhang Chen <dianzhangchen0@xxxxxxxxx>

commit 31a2fbb390fee4231281b939e1979e810f945415 upstream.

The index to access the threads ptrace_bps is controlled by userspace via
syscall: sys_ptrace(), hence leading to a potential exploitation of the
Spectre variant 1 vulnerability.

The index can be controlled from:
    ptrace -> arch_ptrace -> ptrace_get_debugreg.

Fix this by sanitizing the user supplied index before using it access
thread->ptrace_bps.

Signed-off-by: Dianzhang Chen <dianzhangchen0@xxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: bp@xxxxxxxxx
Cc: hpa@xxxxxxxxx
Link: https://lkml.kernel.org/r/1561476617-3759-1-git-send-email-dianzhangchen0@xxxxxxxxx
[bwh: Backported to 3.16: fold in fix-up from commit 223cea6a4f05
 "Merge branch 'x86-pti-for-linus' of ..."]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 arch/x86/kernel/ptrace.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

--- a/arch/x86/kernel/ptrace.c
+++ b/arch/x86/kernel/ptrace.c
@@ -24,6 +24,7 @@
 #include <linux/rcupdate.h>
 #include <linux/export.h>
 #include <linux/context_tracking.h>
+#include <linux/nospec.h>
 
 #include <asm/uaccess.h>
 #include <asm/pgtable.h>
@@ -707,7 +708,8 @@ static unsigned long ptrace_get_debugreg
 	unsigned long val = 0;
 
 	if (n < HBP_NUM) {
-		struct perf_event *bp = thread->ptrace_bps[n];
+		int index = array_index_nospec(n, HBP_NUM);
+		struct perf_event *bp = thread->ptrace_bps[index];
 
 		if (bp)
 			val = bp->hw.info.address;