[PATCH bpf-next v4 2/3] bpf, x86: clean garbage value in the stack of trampoline

[menglong8.dong@xxxxxxxxx]

From: Menglong Dong <imagedong@xxxxxxxxxxx>

There are garbage values in upper bytes when we store the arguments
into stack in save_regs() if the size of the argument less then 8.

As we already reserve 8 byte for the arguments in regs and stack,
it is ok to store/restore the regs in BPF_DW size. Then, the garbage
values in upper bytes will be cleaned.

Signed-off-by: Menglong Dong <imagedong@xxxxxxxxxxx>
---
v4:
- clean grabage value when argument count is 7
---
 arch/x86/net/bpf_jit_comp.c | 45 ++++++++++++++++++++++++++-----------
 1 file changed, 32 insertions(+), 13 deletions(-)

diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index a767e13c8c85..f6f51a5d14db 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -1857,6 +1857,28 @@ st:			if (is_imm8(insn->off))
 	return proglen;
 }
 
+static inline void clean_garbage(u8 **pprog, int nr_regs, int stack_size,
+				 int arg_size)
+{
+	u8 *prog;
+
+	/* clean potential garbage values in upper 32-bit. 'stack_size'
+	 * here is the offset of the 7th argument on-stack.
+	 */
+	if (nr_regs == 7 && arg_size <= 4) {
+		int off = -(stack_size - 4);
+
+		prog = *pprog;
+		/* mov DWORD PTR [rbp + off], 0 */
+		if (!is_imm8(off))
+			EMIT2_off32(0xC7, 0x85, off);
+		else
+			EMIT3(0xC7, 0x45, off);
+		EMIT(0, 4);
+		*pprog = prog;
+	}
+}
+
 static void save_regs(const struct btf_func_model *m, u8 **prog, int nr_regs,
 		      int stack_size)
 {
@@ -1878,8 +1900,7 @@ static void save_regs(const struct btf_func_model *m, u8 **prog, int nr_regs,
 
 		if (i <= 5) {
 			/* copy function arguments from regs into stack */
-			emit_stx(prog, bytes_to_bpf_size(arg_size),
-				 BPF_REG_FP,
+			emit_stx(prog, BPF_DW, BPF_REG_FP,
 				 i == 5 ? X86_REG_R9 : BPF_REG_1 + i,
 				 -(stack_size - i * 8));
 		} else {
@@ -1893,17 +1914,16 @@ static void save_regs(const struct btf_func_model *m, u8 **prog, int nr_regs,
 			 *   8(return addr of the caller)
 			 * which means: rbp + 24
 			 */
-			emit_ldx(prog, bytes_to_bpf_size(arg_size),
-				 BPF_REG_0, BPF_REG_FP,
+			emit_ldx(prog, BPF_DW, BPF_REG_0, BPF_REG_FP,
 				 (i - 6) * 8 + 0x18);
-			emit_stx(prog, bytes_to_bpf_size(arg_size),
-				 BPF_REG_FP,
-				 BPF_REG_0,
+			emit_stx(prog, BPF_DW, BPF_REG_FP, BPF_REG_0,
 				 -(stack_size - i * 8));
 		}
 
 		j = next_same_struct ? j : j + 1;
 	}
+
+	clean_garbage(prog, nr_regs, stack_size - 6 * 8, arg_size);
 }
 
 static void restore_regs(const struct btf_func_model *m, u8 **prog, int nr_regs,
@@ -1925,7 +1945,7 @@ static void restore_regs(const struct btf_func_model *m, u8 **prog, int nr_regs,
 			next_same_struct = !next_same_struct;
 		}
 
-		emit_ldx(prog, bytes_to_bpf_size(arg_size),
+		emit_ldx(prog, BPF_DW,
 			 i == 5 ? X86_REG_R9 : BPF_REG_1 + i,
 			 BPF_REG_FP,
 			 -(stack_size - i * 8));
@@ -1956,17 +1976,16 @@ static void prepare_origin_stack(const struct btf_func_model *m, u8 **prog,
 		}
 
 		if (i > 5) {
-			emit_ldx(prog, bytes_to_bpf_size(arg_size),
-				 BPF_REG_0, BPF_REG_FP,
+			emit_ldx(prog, BPF_DW, BPF_REG_0, BPF_REG_FP,
 				 (i - 6) * 8 + 0x18);
-			emit_stx(prog, bytes_to_bpf_size(arg_size),
-				 BPF_REG_FP,
-				 BPF_REG_0,
+			emit_stx(prog, BPF_DW, BPF_REG_FP, BPF_REG_0,
 				 -(stack_size - (i - 6) * 8));
 		}
 
 		j = next_same_struct ? j : j + 1;
 	}
+
+	clean_garbage(prog, nr_regs, stack_size, arg_size);
 }
 
 static int invoke_bpf_prog(const struct btf_func_model *m, u8 **pprog,
-- 
2.40.1