bpf: fix accessing bpf_sysctl.file_pos on s390

"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 <[email protected]>

---
 include/linux/filter.h                    | 10 ++++++++++
 kernel/bpf/cgroup.c                       |  9 +++++++--
 tools/testing/selftests/bpf/test_sysctl.c |  9 ++++++++-
 3 files changed, 25 insertions(+), 3 deletions(-)

include/linux/filter.h +10 -0

index 92c6e31fb008..94e81c56d81c 100644
@@ -760,6 +760,16 @@ bpf_ctx_narrow_load_shift(u32 off, u32 size, u32 size_default)
 #endif
 }
 
+static inline s16
+bpf_ctx_narrow_access_offset(size_t variable_size, size_t access_size)
+{
+#ifdef __LITTLE_ENDIAN
+	return 0;
+#else
+	return variable_size - access_size;
+#endif
+}
+
 #define bpf_ctx_wide_access_ok(off, size, type, field)			\
 	(size == sizeof(__u64) &&					\
 	off >= offsetof(type, field) &&					\

kernel/bpf/cgroup.c +7 -2

index 0a00eaca6fae..b835fbb13ea8 100644
@@ -1356,7 +1356,9 @@ 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(
+					sizeof(loff_t), sizeof(u32)));
 			*insn++ = BPF_LDX_MEM(
 				BPF_DW, treg, si->dst_reg,
 				offsetof(struct bpf_sysctl_kern, tmp_reg));

@@ -1366,7 +1368,10 @@ static u32 sysctl_convert_ctx_access(enum bpf_access_type type,
 				si->dst_reg, si->src_reg,
 				offsetof(struct bpf_sysctl_kern, ppos));
 			*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(
+					sizeof(loff_t),
+					bpf_size_to_bytes(BPF_SIZE(si->code))));
 		}
 		*target_size = sizeof(u32);
 		break;

tools/testing/selftests/bpf/test_sysctl.c +8 -1

index a3bebd7c68dd..abc26248a7f1 100644
@@ -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];
 

"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 <[email protected]>

---
  include/linux/filter.h                    | 10 ++++++++++
  kernel/bpf/cgroup.c                       |  9 +++++++--
  tools/testing/selftests/bpf/test_sysctl.c |  9 ++++++++-
  3 files changed, 25 insertions(+), 3 deletions(-)

include/linux/filter.h +0 -0

index 92c6e31fb008..94e81c56d81c 100644
@@ -760,6 +760,16 @@ bpf_ctx_narrow_load_shift(u32 off, u32 size, u32 size_default)
  #endif
  }

include/linux/filter.h +1 -0

+
  #define bpf_ctx_wide_access_ok(off, size, type, field)			\
  	(size == sizeof(__u64) &&					\
  	off >= offsetof(type, field) &&					\

kernel/bpf/cgroup.c +7 -2

index 0a00eaca6fae..b835fbb13ea8 100644
@@ -1356,7 +1356,9 @@ 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(
+					sizeof(loff_t), sizeof(u32)));
  			*insn++ = BPF_LDX_MEM(
  				BPF_DW, treg, si->dst_reg,
  				offsetof(struct bpf_sysctl_kern, tmp_reg));

@@ -1366,7 +1368,10 @@ static u32 sysctl_convert_ctx_access(enum bpf_access_type type,
  				si->dst_reg, si->src_reg,
  				offsetof(struct bpf_sysctl_kern, ppos));
  			*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(
+					sizeof(loff_t),
+					bpf_size_to_bytes(BPF_SIZE(si->code))));
  		}
  		*target_size = sizeof(u32);
  		break;

tools/testing/selftests/bpf/test_sysctl.c +8 -1

index a3bebd7c68dd..abc26248a7f1 100644
@@ -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];
 

Am 16.08.2019 um 01:01 schrieb Yonghong Song <[email protected]>:

Quoted 28 lines · Ilya Leoshkevich

"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 <[email protected]>

---
 include/linux/filter.h                    | 10 ++++++++++
 kernel/bpf/cgroup.c                       |  9 +++++++--
 tools/testing/selftests/bpf/test_sysctl.c |  9 ++++++++-
 3 files changed, 25 insertions(+), 3 deletions(-)

include/linux/filter.h +0 -0

index 92c6e31fb008..94e81c56d81c 100644
@@ -760,6 +760,16 @@ bpf_ctx_narrow_load_shift(u32 off, u32 size, u32 size_default)
 #endif
 }

include/linux/filter.h bpf_ctx_narrow_load_shift(u32 off, u32 size, u32 size_default) · line 771

Quoted 10 lines

include/linux/filter.h +9 -0

 
+static inline s16
+bpf_ctx_narrow_access_offset(size_t variable_size, size_t access_size)
+{
+#ifdef __LITTLE_ENDIAN
+	return 0;
+#else
+	return variable_size - access_size;
+#endif
+}

The change looks correct to me.
But now in include/linux/filter.h we have to macros:

static inline u8
bpf_ctx_narrow_load_shift(u32 off, u32 size, u32 size_default)

{
        u8 load_off = off & (size_default - 1);

#ifdef __LITTLE_ENDIAN
        return load_off * 8;
#else
        return (size_default - (load_off + size)) * 8;

#endif
}

static inline s16
bpf_ctx_narrow_access_offset(size_t variable_size, size_t access_size) {

#ifdef __LITTLE_ENDIAN
        return 0;
#else
        return variable_size - access_size;

#endif
}

It would be good if we can have ifdef __LITTLE_ENDIAN only in one place. How about something like below:

static inline u8
bpf_ctx_narrow_access_offset(u32 off, u32 size, u32 size_default)

{
        u8 access_off = off & (size_default - 1);

#ifdef __LITTLE_ENDIAN
        return access_off;
#else
        return size_default - (access_off + size);

#endif
}

static inline u8
bpf_ctx_narrow_load_shift(u32 off, u32 size, u32 size_default)

{
        return bpf_ctx_narrow_access_offset(off, size, size_default) * 8;
}