[PATCH bpf] arm32, bpf: Fix sign-extension mov instruction

[Puranjay Mohan]

The current implementation of the mov instruction with sign extension
clobbers the source register because it sign extends the source and then
moves it to the destination.

Fix this by moving the src to a temporary register before doing the sign
extension only if src is not an emulated register (on the scratch stack).

Also fix the emit_a32_movsx_r64() to put the register back on scratch
stack if that register is emulated on stack.

Fixes: fc832653fa0d ("arm32, bpf: add support for sign-extension mov instruction")
Reported-by: syzbot+186522670e6722692d86@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://lore.kernel.org/all/000000000000e9a8d80615163f2a@xxxxxxxxxx/
Signed-off-by: Puranjay Mohan <puranjay@xxxxxxxxxx>
---
 arch/arm/net/bpf_jit_32.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/arch/arm/net/bpf_jit_32.c b/arch/arm/net/bpf_jit_32.c
index 1d672457d02f..8fde6ab66cb4 100644
--- a/arch/arm/net/bpf_jit_32.c
+++ b/arch/arm/net/bpf_jit_32.c
@@ -878,6 +878,13 @@ static inline void emit_a32_mov_r(const s8 dst, const s8 src, const u8 off,
 
 	rt = arm_bpf_get_reg32(src, tmp[0], ctx);
 	if (off && off != 32) {
+		/* If rt is not a stacked register, move it to tmp, so it doesn't get clobbered by
+		 * the shift operations.
+		 */
+		if (rt == src) {
+			emit(ARM_MOV_R(tmp[0], rt), ctx);
+			rt = tmp[0];
+		}
 		emit(ARM_LSL_I(rt, rt, 32 - off), ctx);
 		emit(ARM_ASR_I(rt, rt, 32 - off), ctx);
 	}
@@ -919,15 +926,15 @@ static inline void emit_a32_movsx_r64(const bool is64, const u8 off, const s8 ds
 	const s8 *tmp = bpf2a32[TMP_REG_1];
 	const s8 *rt;
 
-	rt = arm_bpf_get_reg64(dst, tmp, ctx);
-
 	emit_a32_mov_r(dst_lo, src_lo, off, ctx);
 	if (!is64) {
 		if (!ctx->prog->aux->verifier_zext)
 			/* Zero out high 4 bytes */
 			emit_a32_mov_i(dst_hi, 0, ctx);
 	} else {
+		rt = arm_bpf_get_reg64(dst, tmp, ctx);
 		emit(ARM_ASR_I(rt[0], rt[1], 31), ctx);
+		arm_bpf_put_reg64(dst, rt, ctx);
 	}
 }
 
-- 
2.40.1