[PATCH 5.14 077/172] bpf: Handle return value of BPF_PROG_TYPE_STRUCT_OPS prog

Greg Kroah-Hartman
2021-10-04T13:33:49Z

From: Hou Tao <houtao1@huawei.com>

[ Upstream commit 356ed64991c6847a0c4f2e8fa3b1133f7a14f1fc ]

Currently if a function ptr in struct_ops has a return value, its
caller will get a random return value from it, because the return
value of related BPF_PROG_TYPE_STRUCT_OPS prog is just dropped.

So adding a new flag BPF_TRAMP_F_RET_FENTRY_RET to tell bpf trampoline
to save and return the return value of struct_ops prog if ret_size of
the function ptr is greater than 0. Also restricting the flag to be
used alone.

Fixes: 85d33df357b6 ("bpf: Introduce BPF_MAP_TYPE_STRUCT_OPS")
Signed-off-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20210914023351.3664499-1-houtao1@huawei.com
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
arch/x86/net/bpf_jit_comp.c | 53 ++++++++++++++++++++++++++++---------
include/linux/bpf.h | 2 ++
kernel/bpf/bpf_struct_ops.c | 7 +++--
3 files changed, 47 insertions(+), 15 deletions(-)

diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index 16d76f814e9b..47780844598a 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -1744,7 +1744,7 @@ static void restore_regs(const struct btf_func_model *m, u8 **prog, int nr_args,
}

static int invoke_bpf_prog(const struct btf_func_model *m, u8 **pprog,
- struct bpf_prog *p, int stack_size, bool mod_ret)
+ struct bpf_prog *p, int stack_size, bool save_ret)
{
u8 *prog = *pprog;
u8 *jmp_insn;
@@ -1777,11 +1777,15 @@ static int invoke_bpf_prog(const struct btf_func_model *m, u8 **pprog,
if (emit_call(&prog, p->bpf_func, prog))
return -EINVAL;

- /* BPF_TRAMP_MODIFY_RETURN trampolines can modify the return
+ /*
+ * BPF_TRAMP_MODIFY_RETURN trampolines can modify the return
* of the previous call which is then passed on the stack to
* the next BPF program.
+ *
+ * BPF_TRAMP_FENTRY trampoline may need to return the return
+ * value of BPF_PROG_TYPE_STRUCT_OPS prog.
*/
- if (mod_ret)
+ if (save_ret)
emit_stx(&prog, BPF_DW, BPF_REG_FP, BPF_REG_0, -8);

/* replace 2 nops with JE insn, since jmp target is known */
@@ -1828,13 +1832,15 @@ static int emit_cond_near_jump(u8 **pprog, void *func, void *ip, u8 jmp_cond)
}

static int invoke_bpf(const struct btf_func_model *m, u8 **pprog,
- struct bpf_tramp_progs *tp, int stack_size)
+ struct bpf_tramp_progs *tp, int stack_size,
+ bool save_ret)
{
int i;
u8 *prog = *pprog;

for (i = 0; i < tp->nr_progs; i++) {
- if (invoke_bpf_prog(m, &prog, tp->progs[i], stack_size, false))
+ if (invoke_bpf_prog(m, &prog, tp->progs[i], stack_size,
+ save_ret))
return -EINVAL;
}
*pprog = prog;
@@ -1877,6 +1883,23 @@ static int invoke_bpf_mod_ret(const struct btf_func_model *m, u8 **pprog,
return 0;
}

+static bool is_valid_bpf_tramp_flags(unsigned int flags)
+{
+ if ((flags & BPF_TRAMP_F_RESTORE_REGS) &&
+ (flags & BPF_TRAMP_F_SKIP_FRAME))
+ return false;
+
+ /*
+ * BPF_TRAMP_F_RET_FENTRY_RET is only used by bpf_struct_ops,
+ * and it must be used alone.
+ */
+ if ((flags & BPF_TRAMP_F_RET_FENTRY_RET) &&
+ (flags & ~BPF_TRAMP_F_RET_FENTRY_RET))
+ return false;
+
+ return true;
+}
+
/* Example:
* __be16 eth_type_trans(struct sk_buff *skb, struct net_device *dev);
* its 'struct btf_func_model' will be nr_args=2
@@ -1949,17 +1972,19 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
struct bpf_tramp_progs *fmod_ret = &tprogs[BPF_TRAMP_MODIFY_RETURN];
u8 **branches = NULL;
u8 *prog;
+ bool save_ret;

/* x86-64 supports up to 6 arguments. 7+ can be added in the future */
if (nr_args > 6)
return -ENOTSUPP;

- if ((flags & BPF_TRAMP_F_RESTORE_REGS) &&
- (flags & BPF_TRAMP_F_SKIP_FRAME))
+ if (!is_valid_bpf_tramp_flags(flags))
return -EINVAL;

- if (flags & BPF_TRAMP_F_CALL_ORIG)
- stack_size += 8; /* room for return value of orig_call */
+ /* room for return value of orig_call or fentry prog */
+ save_ret = flags & (BPF_TRAMP_F_CALL_ORIG | BPF_TRAMP_F_RET_FENTRY_RET);
+ if (save_ret)
+ stack_size += 8;

if (flags & BPF_TRAMP_F_SKIP_FRAME)
/* skip patched call instruction and point orig_call to actual
@@ -1986,7 +2011,8 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
}

if (fentry->nr_progs)
- if (invoke_bpf(m, &prog, fentry, stack_size))
+ if (invoke_bpf(m, &prog, fentry, stack_size,
+ flags & BPF_TRAMP_F_RET_FENTRY_RET))
return -EINVAL;

if (fmod_ret->nr_progs) {
@@ -2033,7 +2059,7 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
}

if (fexit->nr_progs)
- if (invoke_bpf(m, &prog, fexit, stack_size)) {
+ if (invoke_bpf(m, &prog, fexit, stack_size, false)) {
ret = -EINVAL;
goto cleanup;
}
@@ -2053,9 +2079,10 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
ret = -EINVAL;
goto cleanup;
}
- /* restore original return value back into RAX */
- emit_ldx(&prog, BPF_DW, BPF_REG_0, BPF_REG_FP, -8);
}
+ /* restore return value of orig_call or fentry prog back into RAX */
+ if (save_ret)
+ emit_ldx(&prog, BPF_DW, BPF_REG_0, BPF_REG_FP, -8);

EMIT1(0x5B); /* pop rbx */
EMIT1(0xC9); /* leave */
diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index e8e2b0393ca9..11da5671d4f0 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -553,6 +553,8 @@ struct btf_func_model {
* programs only. Should not be used with normal calls and indirect calls.
*/
#define BPF_TRAMP_F_SKIP_FRAME BIT(2)
+/* Return the return value of fentry prog. Only used by bpf_struct_ops. */
+#define BPF_TRAMP_F_RET_FENTRY_RET BIT(4)

/* Each call __bpf_prog_enter + call bpf_func + call __bpf_prog_exit is ~50
* bytes on x86. Pick a number to fit into BPF_IMAGE_SIZE / 2
diff --git a/kernel/bpf/bpf_struct_ops.c b/kernel/bpf/bpf_struct_ops.c
index 70f6fd4fa305..2ce17447fb76 100644
--- a/kernel/bpf/bpf_struct_ops.c
+++ b/kernel/bpf/bpf_struct_ops.c
@@ -367,6 +367,7 @@ static int bpf_struct_ops_map_update_elem(struct bpf_map *map, void *key,
const struct btf_type *mtype, *ptype;
struct bpf_prog *prog;
u32 moff;
+ u32 flags;

moff = btf_member_bit_offset(t, member) / 8;
ptype = btf_type_resolve_ptr(btf_vmlinux, member->type, NULL);
@@ -430,10 +431,12 @@ static int bpf_struct_ops_map_update_elem(struct bpf_map *map, void *key,

tprogs[BPF_TRAMP_FENTRY].progs[0] = prog;
tprogs[BPF_TRAMP_FENTRY].nr_progs = 1;
+ flags = st_ops->func_models[i].ret_size > 0 ?
+ BPF_TRAMP_F_RET_FENTRY_RET : 0;
err = arch_prepare_bpf_trampoline(NULL, image,
st_map->image + PAGE_SIZE,
- &st_ops->func_models[i], 0,
- tprogs, NULL);
+ &st_ops->func_models[i],
+ flags, tprogs, NULL);
if (err < 0)
goto reset_unlock;

--
2.33.0