On 8/16/19 12:53 PM, Ilya Leoshkevich wrote:
"ctx:file_pos sysctl:read write ok" fails on s390 with "Read value !=
nux". This is because verifier rewrites a complete 32-bit
bpf_sysctl.file_pos update to a partial update of the first 32 bits of
64-bit *bpf_sysctl_kern.ppos, which is not correct on big-endian
systems.
Fix by using an offset on big-endian systems.
Ditto for bpf_sysctl.file_pos reads. Currently the test does not detect
a problem there, since it expects to see 0, which it gets with high
probability in error cases, so change it to seek to offset 3 and expect
3 in bpf_sysctl.file_pos.
Fixes: e1550bfe0de4 ("bpf: Add file_pos field to bpf_sysctl ctx")
Signed-off-by: Ilya Leoshkevich <iii@xxxxxxxxxxxxx>
---
v1->v2: Merge bpf_ctx_narrow_load_shift and
bpf_ctx_narrow_access_offset.
include/linux/filter.h | 8 ++++----
kernel/bpf/cgroup.c | 10 ++++++++--
kernel/bpf/verifier.c | 4 ++--
tools/testing/selftests/bpf/test_sysctl.c | 9 ++++++++-
4 files changed, 22 insertions(+), 9 deletions(-)
diff --git a/include/linux/filter.h b/include/linux/filter.h
index 92c6e31fb008..2ce57645f3cd 100644
--- a/include/linux/filter.h
+++ b/include/linux/filter.h
@@ -749,14 +749,14 @@ bpf_ctx_narrow_access_ok(u32 off, u32 size, u32 size_default)
}
static inline u8
-bpf_ctx_narrow_load_shift(u32 off, u32 size, u32 size_default)
+bpf_ctx_narrow_access_offset(u32 off, u32 size, u32 size_default)
{
- u8 load_off = off & (size_default - 1);
+ u8 access_off = off & (size_default - 1);
#ifdef __LITTLE_ENDIAN
- return load_off * 8;
+ return access_off;
#else
- return (size_default - (load_off + size)) * 8;
+ return size_default - (access_off + size);
#endif
}
diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
index 0a00eaca6fae..00c4647ce92a 100644
--- a/kernel/bpf/cgroup.c
+++ b/kernel/bpf/cgroup.c
@@ -1325,6 +1325,7 @@ static u32 sysctl_convert_ctx_access(enum bpf_access_type type,
struct bpf_prog *prog, u32 *target_size)
{
struct bpf_insn *insn = insn_buf;
+ u32 read_size;
switch (si->off) {
case offsetof(struct bpf_sysctl, write):
@@ -1356,7 +1357,9 @@ static u32 sysctl_convert_ctx_access(enum bpf_access_type type,
treg, si->dst_reg,
offsetof(struct bpf_sysctl_kern, ppos));
*insn++ = BPF_STX_MEM(
- BPF_SIZEOF(u32), treg, si->src_reg, 0);
+ BPF_SIZEOF(u32), treg, si->src_reg,
+ bpf_ctx_narrow_access_offset(
+ 0, sizeof(u32), sizeof(loff_t)));
*insn++ = BPF_LDX_MEM(
BPF_DW, treg, si->dst_reg,
offsetof(struct bpf_sysctl_kern, tmp_reg));
@@ -1365,8 +1368,11 @@ static u32 sysctl_convert_ctx_access(enum bpf_access_type type,
BPF_FIELD_SIZEOF(struct bpf_sysctl_kern, ppos),
si->dst_reg, si->src_reg,
offsetof(struct bpf_sysctl_kern, ppos));
+ read_size = bpf_size_to_bytes(BPF_SIZE(si->code));
*insn++ = BPF_LDX_MEM(
- BPF_SIZE(si->code), si->dst_reg, si->dst_reg, 0);
+ BPF_SIZE(si->code), si->dst_reg, si->dst_reg,
+ bpf_ctx_narrow_access_offset(
+ 0, read_size, sizeof(loff_t)));
I see what you're doing, but generally I'm a bit puzzled on why we need these
partial store/loads and cannot access the full loff_t value internally with the
rewrite. Why was BPF_SIZEOF(u32) chosen in the first place? Looks like git history
doesn't have any useful insight here ... Andrey mind to put some clarifications
on this? Thx
}
*target_size = sizeof(u32);
break;
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index c84d83f86141..d1d4c995a9eb 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -8616,8 +8616,8 @@ static int convert_ctx_accesses(struct bpf_verifier_env *env)
}
if (is_narrower_load && size < target_size) {
- u8 shift = bpf_ctx_narrow_load_shift(off, size,
- size_default);
+ u8 shift = bpf_ctx_narrow_access_offset(
+ off, size, size_default) * 8;
if (ctx_field_size <= 4) {
if (shift)
insn_buf[cnt++] = BPF_ALU32_IMM(BPF_RSH,
diff --git a/tools/testing/selftests/bpf/test_sysctl.c b/tools/testing/selftests/bpf/test_sysctl.c
index a3bebd7c68dd..abc26248a7f1 100644
--- a/tools/testing/selftests/bpf/test_sysctl.c
+++ b/tools/testing/selftests/bpf/test_sysctl.c
@@ -31,6 +31,7 @@ struct sysctl_test {
enum bpf_attach_type attach_type;
const char *sysctl;
int open_flags;
+ int seek;
const char *newval;
const char *oldval;
enum {
@@ -139,7 +140,7 @@ static struct sysctl_test tests[] = {
/* If (file_pos == X) */
BPF_LDX_MEM(BPF_W, BPF_REG_7, BPF_REG_1,
offsetof(struct bpf_sysctl, file_pos)),
- BPF_JMP_IMM(BPF_JNE, BPF_REG_7, 0, 2),
+ BPF_JMP_IMM(BPF_JNE, BPF_REG_7, 3, 2),
/* return ALLOW; */
BPF_MOV64_IMM(BPF_REG_0, 1),
@@ -152,6 +153,7 @@ static struct sysctl_test tests[] = {
.attach_type = BPF_CGROUP_SYSCTL,
.sysctl = "kernel/ostype",
.open_flags = O_RDONLY,
+ .seek = 3,
.result = SUCCESS,
},
{
@@ -1442,6 +1444,11 @@ static int access_sysctl(const char *sysctl_path,
if (fd < 0)
return fd;
+ if (test->seek && lseek(fd, test->seek, SEEK_SET) == -1) {
+ log_err("lseek(%d) failed", test->seek);
+ goto err;
+ }
+
if (test->open_flags == O_RDONLY) {
char buf[128];
