arm64: fix possible spectre-v1 write in ptrace_hbp_set_event()

It's possible for userspace to control idx. Sanitize idx when using it
as an array index, to inhibit the potential spectre-v1 write gadget.

Found by smatch.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
index 379b2b7..299758f 100644 (file)
--- a/arch/arm64/kernel/ptrace.c
+++ b/arch/arm64/kernel/ptrace.c
@@ -277,19 +277,22 @@ static int ptrace_hbp_set_event(unsigned int note_type,
 
        switch (note_type) {
        case NT_ARM_HW_BREAK:
-               if (idx < ARM_MAX_BRP) {
-                       tsk->thread.debug.hbp_break[idx] = bp;
-                       err = 0;
-               }
+               if (idx >= ARM_MAX_BRP)
+                       goto out;
+               idx = array_index_nospec(idx, ARM_MAX_BRP);
+               tsk->thread.debug.hbp_break[idx] = bp;
+               err = 0;
                break;
        case NT_ARM_HW_WATCH:
-               if (idx < ARM_MAX_WRP) {
-                       tsk->thread.debug.hbp_watch[idx] = bp;
-                       err = 0;
-               }
+               if (idx >= ARM_MAX_WRP)
+                       goto out;
+               idx = array_index_nospec(idx, ARM_MAX_WRP);
+               tsk->thread.debug.hbp_watch[idx] = bp;
+               err = 0;
                break;
        }
 
+out:
        return err;
 }