net: filter: make JITs zero A for SKF_AD_ALU_XOR_X

The SKF_AD_ALU_XOR_X ancillary is not like the other ancillary data
instructions since it XORs A with X while all the others replace A with
some loaded value.  All the BPF JITs fail to clear A if this is used as
the first instruction in a filter.  This was found using american fuzzy
lop.

Add a helper to determine if A needs to be cleared given the first
instruction in a filter, and use this in the JITs.  Except for ARM, the
rest have only been compile-tested.

Fixes: 3480593131e0 ("net: filter: get rid of BPF_S_* enum")
Signed-off-by: Rabin Vincent <rabin@rab.in>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

5 files changed, 25 insertions, 56 deletions

diff --git a/arch/arm/net/bpf_jit_32.c b/arch/arm/net/bpf_jit_32.c
index 591f9db3bf40..e153eb065fe4 100644
--- a/arch/arm/net/bpf_jit_32.c
+++ b/arch/arm/net/bpf_jit_32.c
@@ -187,19 +187,6 @@ static inline int mem_words_used(struct jit_ctx *ctx)
         return fls(ctx->seen & SEEN_MEM);
 }
 
-static inline bool is_load_to_a(u16 inst)
-{
-        switch (inst) {
-        case BPF_LD | BPF_W | BPF_LEN:
-        case BPF_LD | BPF_W | BPF_ABS:
-        case BPF_LD | BPF_H | BPF_ABS:
-        case BPF_LD | BPF_B | BPF_ABS:
-                return true;
-        default:
-                return false;
-        }
-}
-
 static void jit_fill_hole(void *area, unsigned int size)
 {
         u32 *ptr;
@@ -211,7 +198,6 @@ static void jit_fill_hole(void *area, unsigned int size)
 static void build_prologue(struct jit_ctx *ctx)
 {
         u16 reg_set = saved_regs(ctx);
-        u16 first_inst = ctx->skf->insns[0].code;
         u16 off;
 
 #ifdef CONFIG_FRAME_POINTER
@@ -241,7 +227,7 @@ static void build_prologue(struct jit_ctx *ctx)
                 emit(ARM_MOV_I(r_X, 0), ctx);
 
         /* do not leak kernel data to userspace */
-        if ((first_inst != (BPF_RET | BPF_K)) && !(is_load_to_a(first_inst)))
+        if (bpf_needs_clear_a(&ctx->skf->insns[0]))
                 emit(ARM_MOV_I(r_A, 0), ctx);
 
         /* stack space for the BPF_MEM words */
diff --git a/arch/mips/net/bpf_jit.c b/arch/mips/net/bpf_jit.c
index 77cb27309db2..1a8c96035716 100644
--- a/arch/mips/net/bpf_jit.c
+++ b/arch/mips/net/bpf_jit.c
@@ -521,19 +521,6 @@ static inline u16 align_sp(unsigned int num)
         return num;
 }
 
-static bool is_load_to_a(u16 inst)
-{
-        switch (inst) {
-        case BPF_LD | BPF_W | BPF_LEN:
-        case BPF_LD | BPF_W | BPF_ABS:
-        case BPF_LD | BPF_H | BPF_ABS:
-        case BPF_LD | BPF_B | BPF_ABS:
-                return true;
-        default:
-                return false;
-        }
-}
-
 static void save_bpf_jit_regs(struct jit_ctx *ctx, unsigned offset)
 {
         int i = 0, real_off = 0;
@@ -614,7 +601,6 @@ static unsigned int get_stack_depth(struct jit_ctx *ctx)
 
 static void build_prologue(struct jit_ctx *ctx)
 {
-        u16 first_inst = ctx->skf->insns[0].code;
         int sp_off;
 
         /* Calculate the total offset for the stack pointer */
@@ -641,7 +627,7 @@ static void build_prologue(struct jit_ctx *ctx)
                 emit_jit_reg_move(r_X, r_zero, ctx);
 
         /* Do not leak kernel data to userspace */
-        if ((first_inst != (BPF_RET | BPF_K)) && !(is_load_to_a(first_inst)))
+        if (bpf_needs_clear_a(&ctx->skf->insns[0]))
                 emit_jit_reg_move(r_A, r_zero, ctx);
 }
 
diff --git a/arch/powerpc/net/bpf_jit_comp.c b/arch/powerpc/net/bpf_jit_comp.c
index 04782164ee67..2d66a8446198 100644
--- a/arch/powerpc/net/bpf_jit_comp.c
+++ b/arch/powerpc/net/bpf_jit_comp.c
@@ -78,18 +78,9 @@ static void bpf_jit_build_prologue(struct bpf_prog *fp, u32 *image,
                 PPC_LI(r_X, 0);
         }
 
-        switch (filter[0].code) {
-        case BPF_RET | BPF_K:
-        case BPF_LD | BPF_W | BPF_LEN:
-        case BPF_LD | BPF_W | BPF_ABS:
-        case BPF_LD | BPF_H | BPF_ABS:
-        case BPF_LD | BPF_B | BPF_ABS:
-                /* first instruction sets A register (or is RET 'constant') */
-                break;
-        default:
-                /* make sure we dont leak kernel information to user */
+        /* make sure we dont leak kernel information to user */
+        if (bpf_needs_clear_a(&filter[0]))
                 PPC_LI(r_A, 0);
-        }
 }
 
 static void bpf_jit_build_epilogue(u32 *image, struct codegen_context *ctx)
diff --git a/arch/sparc/net/bpf_jit_comp.c b/arch/sparc/net/bpf_jit_comp.c
index 22564f5f2364..3e6e05a7c4c2 100644
--- a/arch/sparc/net/bpf_jit_comp.c
+++ b/arch/sparc/net/bpf_jit_comp.c
@@ -420,22 +420,9 @@ void bpf_jit_compile(struct bpf_prog *fp)
                 }
                 emit_reg_move(O7, r_saved_O7);
 
-                switch (filter[0].code) {
-                case BPF_RET | BPF_K:
-                case BPF_LD | BPF_W | BPF_LEN:
-                case BPF_LD | BPF_W | BPF_ABS:
-                case BPF_LD | BPF_H | BPF_ABS:
-                case BPF_LD | BPF_B | BPF_ABS:
-                        /* The first instruction sets the A register (or is
-                         * a "RET 'constant'")
-                         */
-                        break;
-                default:
-                        /* Make sure we dont leak kernel information to the
-                         * user.
-                         */
+                /* Make sure we dont leak kernel information to the user. */
+                if (bpf_needs_clear_a(&filter[0]))
                         emit_clear(r_A); /* A = 0 */
-                }
 
                 for (i = 0; i < flen; i++) {
                         unsigned int K = filter[i].k;
diff --git a/include/linux/filter.h b/include/linux/filter.h
index 4165e9ac9e36..5972ffe5719a 100644
--- a/include/linux/filter.h
+++ b/include/linux/filter.h
@@ -493,6 +493,25 @@ static inline void bpf_jit_free(struct bpf_prog *fp)
 
 #define BPF_ANC         BIT(15)
 
+static inline bool bpf_needs_clear_a(const struct sock_filter *first)
+{
+        switch (first->code) {
+        case BPF_RET | BPF_K:
+        case BPF_LD | BPF_W | BPF_LEN:
+                return false;
+
+        case BPF_LD | BPF_W | BPF_ABS:
+        case BPF_LD | BPF_H | BPF_ABS:
+        case BPF_LD | BPF_B | BPF_ABS:
+                if (first->k == SKF_AD_OFF + SKF_AD_ALU_XOR_X)
+                        return true;
+                return false;
+
+        default:
+                return true;
+        }
+}
+
 static inline u16 bpf_anc_helper(const struct sock_filter *ftest)
 {
         BUG_ON(ftest->code & BPF_ANC);