ARM: kprobes: Reject probing of LDR/STR instructions which update PC unpredictably

Using PC as an base register with writeback is UNPREDICTABLE, as is non
word-sized loads or stores of PC. (We only really care about preventing
loads to PC but it keeps the code simpler if we also exclude stores.)

Signed-off-by: Jon Medhurst <tixy@yxit.co.uk>
Signed-off-by: Nicolas Pitre <nicolas.pitre@linaro.org>

1 files changed, 17 insertions, 0 deletions

diff --git a/arch/arm/kernel/kprobes-decode.c b/arch/arm/kernel/kprobes-decode.c
index 348ff47acd06..a4dba1f7c87b 100644
--- a/arch/arm/kernel/kprobes-decode.c
+++ b/arch/arm/kernel/kprobes-decode.c
@@ -70,6 +70,12 @@
 
 #define is_r15(insn, bitpos) (((insn) & (0xf << bitpos)) == (0xf << bitpos))
 
+/*
+ * Test if load/store instructions writeback the address register.
+ * if P (bit 24) == 0 or W (bit 21) == 1
+ */
+#define is_writeback(insn) ((insn ^ 0x01000000) & 0x01200000)
+
 #define PSR_fs  (PSR_f|PSR_s)
 
 #define KPROBE_RETURN_INSTRUCTION       0xe1a0f00e      /* mov pc, lr */
@@ -886,6 +892,9 @@ prep_emulate_ldr_str(kprobe_opcode_t insn, struct arch_specific_insn *asi)
         int not_imm = (insn & (1 << 26)) ? (insn & (1 << 25))
                                          : (~insn & (1 << 22));
 
+        if (is_writeback(insn) && is_r15(insn, 16))
+                return INSN_REJECTED;   /* Writeback to PC */
+
         insn &= 0xfff00fff;
         insn |= 0x00001000;     /* Rn = r0, Rd = r1 */
         if (not_imm) {
@@ -1167,6 +1176,11 @@ space_cccc_000x(kprobe_opcode_t insn, struct arch_specific_insn *asi)
 
                 } else if ((insn & 0x0e1000d0) == 0x00000d0) {
                         /* STRD/LDRD */
+                        if ((insn & 0x0000e000) == 0x0000e000)
+                                return INSN_REJECTED;   /* Rd is LR or PC */
+                        if (is_writeback(insn) && is_r15(insn, 16))
+                                return INSN_REJECTED;   /* Writeback to PC */
+
                         insn &= 0xfff00fff;
                         insn |= 0x00002000;     /* Rn = r0, Rd = r2 */
                         if (insn & (1 << 22)) {
@@ -1180,6 +1194,9 @@ space_cccc_000x(kprobe_opcode_t insn, struct arch_specific_insn *asi)
                         return INSN_GOOD;
                 }
 
+                /* LDRH/STRH/LDRSB/LDRSH */
+                if (is_r15(insn, 12))
+                        return INSN_REJECTED;   /* Rd is PC */
                 return prep_emulate_ldr_str(insn, asi);
         }