bpf, x86: add support for indirect jumps

Add support for a new instruction

    BPF_JMP|BPF_X|BPF_JA, SRC=0, DST=Rx, off=0, imm=0

which does an indirect jump to a location stored in Rx.  The register
Rx should have type PTR_TO_INSN. This new type assures that the Rx
register contains a value (or a range of values) loaded from a
correct jump table – map of type instruction array.

For example, for a C switch LLVM will generate the following code:

    0:   r3 = r1                    # "switch (r3)"
    1:   if r3 > 0x13 goto +0x666   # check r3 boundaries
    2:   r3 <<= 0x3                 # adjust to an index in array of addresses
    3:   r1 = 0xbeef ll             # r1 is PTR_TO_MAP_VALUE, r1->map_ptr=M
    5:   r1 += r3                   # r1 inherits boundaries from r3
    6:   r1 = *(u64 *)(r1 + 0x0)    # r1 now has type INSN_TO_PTR
    7:   gotox r1                   # jit will generate proper code

Here the gotox instruction corresponds to one particular map. This is
possible however to have a gotox instruction which can be loaded from
different maps, e.g.

    0:   r1 &= 0x1
    1:   r2 <<= 0x3
    2:   r3 = 0x0 ll                # load from map M_1
    4:   r3 += r2
    5:   if r1 == 0x0 goto +0x4
    6:   r1 <<= 0x3
    7:   r3 = 0x0 ll                # load from map M_2
    9:   r3 += r1
    A:   r1 = *(u64 *)(r3 + 0x0)
    B:   gotox r1                   # jump to target loaded from M_1 or M_2

During check_cfg stage the verifier will collect all the maps which
point to inside the subprog being verified. When building the config,
the high 16 bytes of the insn_state are used, so this patch
(theoretically) supports jump tables of up to 2^16 slots.

During the later stage, in check_indirect_jump, it is checked that
the register Rx was loaded from a particular instruction array.

Signed-off-by: Anton Protopopov <a.s.protopopov@gmail.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20251105090410.1250500-9-a.s.protopopov@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

2 files changed, 10 insertions, 0 deletions

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 9d41a6affcef..09d5dc541d1c 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -1001,6 +1001,7 @@ enum bpf_reg_type {
 	PTR_TO_ARENA,
 	PTR_TO_BUF,		 /* reg points to a read/write buffer */
 	PTR_TO_FUNC,		 /* reg points to a bpf program function */
+	PTR_TO_INSN,		 /* reg points to a bpf program instruction */
 	CONST_PTR_TO_DYNPTR,	 /* reg points to a const struct bpf_dynptr */
 	__BPF_REG_TYPE_MAX,
 
diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
index 6b820d8d77af..5441341f1ab9 100644
--- a/include/linux/bpf_verifier.h
+++ b/include/linux/bpf_verifier.h
@@ -527,6 +527,7 @@ struct bpf_insn_aux_data {
 		struct {
 			u32 map_index;		/* index into used_maps[] */
 			u32 map_off;		/* offset from value base address */
+			struct bpf_iarray *jt;	/* jump table for gotox instruction */
 		};
 		struct {
 			enum bpf_reg_type reg_type;	/* type of pseudo_btf_id */
@@ -840,6 +841,7 @@ struct bpf_verifier_env {
 	struct bpf_scc_info **scc_info;
 	u32 scc_cnt;
 	struct bpf_iarray *succ;
+	struct bpf_iarray *gotox_tmp_buf;
 };
 
 static inline struct bpf_func_info_aux *subprog_aux(struct bpf_verifier_env *env, int subprog)
@@ -1050,6 +1052,13 @@ static inline bool bpf_stack_narrow_access_ok(int off, int fill_size, int spill_
 	return !(off % BPF_REG_SIZE);
 }
 
+static inline bool insn_is_gotox(struct bpf_insn *insn)
+{
+	return BPF_CLASS(insn->code) == BPF_JMP &&
+	       BPF_OP(insn->code) == BPF_JA &&
+	       BPF_SRC(insn->code) == BPF_X;
+}
+
 const char *reg_type_str(struct bpf_verifier_env *env, enum bpf_reg_type type);
 const char *dynptr_type_str(enum bpf_dynptr_type type);
 const char *iter_type_str(const struct btf *btf, u32 btf_id);