[PATCH bpf v4 0/5] Fixes for stack with allow_ptr_leaks

[PATCH bpf v4 0/5] Fixes for stack with allow_ptr_leaks

[Kumar Kartikeya Dwivedi]

Two fixes for usability/correctness gaps when interacting with the stack
without CAP_PERFMON (i.e. with allow_ptr_leaks = false). See the commits
for details. I've verified that the tests fail when run without the fixes.

Changelog:
----------
v3 -> v4
v3: https://lore.kernel.org/bpf/20241202083814.1888784-1-memxor@gmail.com

 * Address Andrii's comments
   * Fix bug paperered over by missing CAP_NET_ADMIN in verifier_mtu
     test
   * Add warning when undefined CAP_ constant is specified, and fail
     test
   * Reorder annotations to be more clear
   * Verify that fixes fail without patches again
 * Add Acked-by from Andrii

v2 -> v3
v2: https://lore.kernel.org/bpf/20241127212026.3580542-1-memxor@gmail.com

 * Address comments from Eduard
   * Fix comment for mark_stack_slot_misc
   * We can simply always return early when stype == STACK_INVALID
   * Drop allow_ptr_leaks conditionals
   * Add Eduard's __caps_unpriv patch into the series
   * Convert test_verifier_mtu to use it
   * Move existing tests to __caps_unpriv annotation and verifier_spill_fill.c
   * Add Acked-by from Eduard

v1 -> v2
v1: https://lore.kernel.org/bpf/20241127185135.2753982-1-memxor@gmail.com

 * Fix CI errors in selftest by removing dependence on BPF_ST

Eduard Zingerman (1):
  selftests/bpf: Introduce __caps_unpriv annotation for tests

Kumar Kartikeya Dwivedi (3):
  bpf: Don't mark STACK_INVALID as STACK_MISC in mark_stack_slot_misc
  selftests/bpf: Add test for reading from STACK_INVALID slots
  selftests/bpf: Add test for narrow spill into 64-bit spilled scalar

Tao Lyu (1):
  bpf: Fix narrow scalar spill onto 64-bit spilled scalar slots

 kernel/bpf/verifier.c                         | 10 ++--
 .../selftests/bpf/prog_tests/verifier.c       | 19 +-------
 tools/testing/selftests/bpf/progs/bpf_misc.h  | 12 +++++
 .../selftests/bpf/progs/verifier_mtu.c        |  4 +-
 .../selftests/bpf/progs/verifier_spill_fill.c | 35 ++++++++++++++
 tools/testing/selftests/bpf/test_loader.c     | 46 +++++++++++++++++++
 6 files changed, 104 insertions(+), 22 deletions(-)


base-commit: 5a6ea7022ff4d2a65ae328619c586d6a8909b48b
-- 
2.43.5

[PATCH bpf v4 1/5] bpf: Don't mark STACK_INVALID as STACK_MISC in mark_stack_slot_misc

[Kumar Kartikeya Dwivedi]

Inside mark_stack_slot_misc, we should not upgrade STACK_INVALID to
STACK_MISC when allow_ptr_leaks is false, since invalid contents
shouldn't be read unless the program has the relevant capabilities.
The relaxation only makes sense when env->allow_ptr_leaks is true.

However, such conversion in privileged mode becomes unnecessary, as
invalid slots can be read without being upgraded to STACK_MISC.

Currently, the condition is inverted (i.e. checking for true instead of
false), simply remove it to restore correct behavior.

Fixes: eaf18febd6eb ("bpf: preserve STACK_ZERO slots on partial reg spills")
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Reported-by: Tao Lyu <tao.lyu@epfl.ch>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 kernel/bpf/verifier.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 2fd35465d650..f18aad339de8 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -1202,14 +1202,17 @@ static bool is_spilled_scalar_reg64(const struct bpf_stack_state *stack)
 /* Mark stack slot as STACK_MISC, unless it is already STACK_INVALID, in which
  * case they are equivalent, or it's STACK_ZERO, in which case we preserve
  * more precise STACK_ZERO.
- * Note, in uprivileged mode leaving STACK_INVALID is wrong, so we take
- * env->allow_ptr_leaks into account and force STACK_MISC, if necessary.
+ * Regardless of allow_ptr_leaks setting (i.e., privileged or unprivileged
+ * mode), we won't promote STACK_INVALID to STACK_MISC. In privileged case it is
+ * unnecessary as both are considered equivalent when loading data and pruning,
+ * in case of unprivileged mode it will be incorrect to allow reads of invalid
+ * slots.
  */
 static void mark_stack_slot_misc(struct bpf_verifier_env *env, u8 *stype)
 {
 	if (*stype == STACK_ZERO)
 		return;
-	if (env->allow_ptr_leaks && *stype == STACK_INVALID)
+	if (*stype == STACK_INVALID)
 		return;
 	*stype = STACK_MISC;
 }
-- 
2.43.5

[PATCH bpf v4 2/5] bpf: Fix narrow scalar spill onto 64-bit spilled scalar slots

[Kumar Kartikeya Dwivedi]

From: Tao Lyu <tao.lyu@epfl.ch>

When CAP_PERFMON and CAP_SYS_ADMIN (allow_ptr_leaks) are disabled, the
verifier aims to reject partial overwrite on an 8-byte stack slot that
contains a spilled pointer.

However, in such a scenario, it rejects all partial stack overwrites as
long as the targeted stack slot is a spilled register, because it does
not check if the stack slot is a spilled pointer.

Incomplete checks will result in the rejection of valid programs, which
spill narrower scalar values onto scalar slots, as shown below.

0: R1=ctx() R10=fp0
; asm volatile ( @ repro.bpf.c:679
0: (7a) *(u64 *)(r10 -8) = 1          ; R10=fp0 fp-8_w=1
1: (62) *(u32 *)(r10 -8) = 1
attempt to corrupt spilled pointer on stack
processed 2 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0.

Fix this by expanding the check to not consider spilled scalar registers
when rejecting the write into the stack.

Previous discussion on this patch is at link [0].

  [0]: https://lore.kernel.org/bpf/20240403202409.2615469-1-tao.lyu@epfl.ch

Fixes: ab125ed3ec1c ("bpf: fix check for attempt to corrupt spilled pointer")
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Tao Lyu <tao.lyu@epfl.ch>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 kernel/bpf/verifier.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index f18aad339de8..01fbef9576e0 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -4703,6 +4703,7 @@ static int check_stack_write_fixed_off(struct bpf_verifier_env *env,
 	 */
 	if (!env->allow_ptr_leaks &&
 	    is_spilled_reg(&state->stack[spi]) &&
+	    !is_spilled_scalar_reg(&state->stack[spi]) &&
 	    size != BPF_REG_SIZE) {
 		verbose(env, "attempt to corrupt spilled pointer on stack\n");
 		return -EACCES;
-- 
2.43.5

[PATCH bpf v4 3/5] selftests/bpf: Introduce __caps_unpriv annotation for tests

[Kumar Kartikeya Dwivedi]

From: Eduard Zingerman <eddyz87@gmail.com>

Add a __caps_unpriv annotation so that tests requiring specific
capabilities while dropping the rest can conveniently specify them
during selftest declaration instead of munging with capabilities at
runtime from the testing binary.

While at it, let us convert test_verifier_mtu to use this new support
instead.

Since we do not want to include linux/capability.h, we only defined the
four main capabilities BPF subsystem deals with in bpf_misc.h for use in
tests. If the user passes a CAP_SYS_NICE or anything else that's not
defined in the header, capability parsing code will return a warning.

Also reject strtol returning 0. CAP_CHOWN = 0 but we'll never need to
use it, and strtol doesn't errno on failed conversion. Fail the test in
such a case.

The original diff for this idea is available at link [0].

  [0]: https://lore.kernel.org/bpf/a1e48f5d9ae133e19adc6adf27e19d585e06bab4.camel@gmail.com

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
[ Kartikeya: rebase on bpf-next, add warn to parse_caps, convert test_verifier_mtu ]
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 .../selftests/bpf/prog_tests/verifier.c       | 19 +-------
 tools/testing/selftests/bpf/progs/bpf_misc.h  | 12 +++++
 .../selftests/bpf/progs/verifier_mtu.c        |  4 +-
 tools/testing/selftests/bpf/test_loader.c     | 46 +++++++++++++++++++
 4 files changed, 62 insertions(+), 19 deletions(-)

diff --git a/tools/testing/selftests/bpf/prog_tests/verifier.c b/tools/testing/selftests/bpf/prog_tests/verifier.c
index d9f65adb456b..3ee40ee9413a 100644
--- a/tools/testing/selftests/bpf/prog_tests/verifier.c
+++ b/tools/testing/selftests/bpf/prog_tests/verifier.c
@@ -225,24 +225,7 @@ void test_verifier_xdp(void)                  { RUN(verifier_xdp); }
 void test_verifier_xdp_direct_packet_access(void) { RUN(verifier_xdp_direct_packet_access); }
 void test_verifier_bits_iter(void) { RUN(verifier_bits_iter); }
 void test_verifier_lsm(void)                  { RUN(verifier_lsm); }
-
-void test_verifier_mtu(void)
-{
-	__u64 caps = 0;
-	int ret;
-
-	/* In case CAP_BPF and CAP_PERFMON is not set */
-	ret = cap_enable_effective(1ULL << CAP_BPF | 1ULL << CAP_NET_ADMIN, &caps);
-	if (!ASSERT_OK(ret, "set_cap_bpf_cap_net_admin"))
-		return;
-	ret = cap_disable_effective(1ULL << CAP_SYS_ADMIN | 1ULL << CAP_PERFMON, NULL);
-	if (!ASSERT_OK(ret, "disable_cap_sys_admin"))
-		goto restore_cap;
-	RUN(verifier_mtu);
-restore_cap:
-	if (caps)
-		cap_enable_effective(caps, NULL);
-}
+void test_verifier_mtu(void)		      { RUN(verifier_mtu); }
 
 static int init_test_val_map(struct bpf_object *obj, char *map_name)
 {
diff --git a/tools/testing/selftests/bpf/progs/bpf_misc.h b/tools/testing/selftests/bpf/progs/bpf_misc.h
index eccaf955e394..f45f4352feeb 100644
--- a/tools/testing/selftests/bpf/progs/bpf_misc.h
+++ b/tools/testing/selftests/bpf/progs/bpf_misc.h
@@ -5,6 +5,10 @@
 #define XSTR(s) STR(s)
 #define STR(s) #s
 
+/* Expand a macro and then stringize the expansion */
+#define QUOTE(str) #str
+#define EXPAND_QUOTE(str) QUOTE(str)
+
 /* This set of attributes controls behavior of the
  * test_loader.c:test_loader__run_subtests().
  *
@@ -106,6 +110,7 @@
  * __arch_*          Specify on which architecture the test case should be tested.
  *                   Several __arch_* annotations could be specified at once.
  *                   When test case is not run on current arch it is marked as skipped.
+ * __caps_unpriv     Specify the capabilities that should be set when running the test.
  */
 #define __msg(msg)		__attribute__((btf_decl_tag("comment:test_expect_msg=" XSTR(__COUNTER__) "=" msg)))
 #define __xlated(msg)		__attribute__((btf_decl_tag("comment:test_expect_xlated=" XSTR(__COUNTER__) "=" msg)))
@@ -129,6 +134,13 @@
 #define __arch_x86_64		__arch("X86_64")
 #define __arch_arm64		__arch("ARM64")
 #define __arch_riscv64		__arch("RISCV64")
+#define __caps_unpriv(caps)	__attribute__((btf_decl_tag("comment:test_caps_unpriv=" EXPAND_QUOTE(caps))))
+
+/* Define common capabilities tested using __caps_unpriv */
+#define CAP_NET_ADMIN		12
+#define CAP_SYS_ADMIN		21
+#define CAP_PERFMON		38
+#define CAP_BPF			39
 
 /* Convenience macro for use with 'asm volatile' blocks */
 #define __naked __attribute__((naked))
diff --git a/tools/testing/selftests/bpf/progs/verifier_mtu.c b/tools/testing/selftests/bpf/progs/verifier_mtu.c
index 70c7600a26a0..4ccf1ebc42d1 100644
--- a/tools/testing/selftests/bpf/progs/verifier_mtu.c
+++ b/tools/testing/selftests/bpf/progs/verifier_mtu.c
@@ -6,7 +6,9 @@
 
 SEC("tc/ingress")
 __description("uninit/mtu: write rejected")
-__failure __msg("invalid indirect read from stack")
+__success
+__caps_unpriv(CAP_BPF|CAP_NET_ADMIN)
+__failure_unpriv __msg_unpriv("invalid indirect read from stack")
 int tc_uninit_mtu(struct __sk_buff *ctx)
 {
 	__u32 mtu;
diff --git a/tools/testing/selftests/bpf/test_loader.c b/tools/testing/selftests/bpf/test_loader.c
index 3e9b009580d4..53b06647cf57 100644
--- a/tools/testing/selftests/bpf/test_loader.c
+++ b/tools/testing/selftests/bpf/test_loader.c
@@ -36,6 +36,7 @@
 #define TEST_TAG_ARCH "comment:test_arch="
 #define TEST_TAG_JITED_PFX "comment:test_jited="
 #define TEST_TAG_JITED_PFX_UNPRIV "comment:test_jited_unpriv="
+#define TEST_TAG_CAPS_UNPRIV "comment:test_caps_unpriv="
 
 /* Warning: duplicated in bpf_misc.h */
 #define POINTER_VALUE	0xcafe4all
@@ -74,6 +75,7 @@ struct test_subspec {
 	struct expected_msgs jited;
 	int retval;
 	bool execute;
+	__u64 caps;
 };
 
 struct test_spec {
@@ -276,6 +278,37 @@ static int parse_int(const char *str, int *val, const char *name)
 	return 0;
 }
 
+static int parse_caps(const char *str, __u64 *val, const char *name)
+{
+	int cap_flag = 0;
+	char *token = NULL, *saveptr = NULL;
+
+	char *str_cpy = strdup(str);
+	if (str_cpy == NULL) {
+		PRINT_FAIL("Memory allocation failed\n");
+		return -EINVAL;
+	}
+
+	token = strtok_r(str_cpy, "|", &saveptr);
+	while (token != NULL) {
+		errno = 0;
+		if (!strncmp("CAP_", token, sizeof("CAP_") - 1)) {
+			PRINT_FAIL("define %s constant in bpf_misc.h, failed to parse caps\n", token);
+			return -EINVAL;
+		}
+		cap_flag = strtol(token, NULL, 10);
+		if (!cap_flag || errno) {
+			PRINT_FAIL("failed to parse caps %s\n", name);
+			return -EINVAL;
+		}
+		*val |= (1ULL << cap_flag);
+		token = strtok_r(NULL, "|", &saveptr);
+	}
+
+	free(str_cpy);
+	return 0;
+}
+
 static int parse_retval(const char *str, int *val, const char *name)
 {
 	struct {
@@ -541,6 +574,12 @@ static int parse_test_spec(struct test_loader *tester,
 			jit_on_next_line = true;
 		} else if (str_has_pfx(s, TEST_BTF_PATH)) {
 			spec->btf_custom_path = s + sizeof(TEST_BTF_PATH) - 1;
+		} else if (str_has_pfx(s, TEST_TAG_CAPS_UNPRIV)) {
+			val = s + sizeof(TEST_TAG_CAPS_UNPRIV) - 1;
+			err = parse_caps(val, &spec->unpriv.caps, "test caps");
+			if (err)
+				goto cleanup;
+			spec->mode_mask |= UNPRIV;
 		}
 	}
 
@@ -917,6 +956,13 @@ void run_subtest(struct test_loader *tester,
 			test__end_subtest();
 			return;
 		}
+		if (subspec->caps) {
+			err = cap_enable_effective(subspec->caps, NULL);
+			if (err) {
+				PRINT_FAIL("failed to set capabilities: %i, %s\n", err, strerror(err));
+				goto subtest_cleanup;
+			}
+		}
 	}
 
 	/* Implicitly reset to NULL if next test case doesn't specify */
-- 
2.43.5

[PATCH bpf v4 4/5] selftests/bpf: Add test for reading from STACK_INVALID slots

[Kumar Kartikeya Dwivedi]

Ensure that when CAP_PERFMON is dropped, and the verifier sees
allow_ptr_leaks as false, we are not permitted to read from a
STACK_INVALID slot. Without the fix, the test will report unexpected
success in loading.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 .../selftests/bpf/progs/verifier_spill_fill.c  | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/tools/testing/selftests/bpf/progs/verifier_spill_fill.c b/tools/testing/selftests/bpf/progs/verifier_spill_fill.c
index 671d9f415dbf..bab6d789ba00 100644
--- a/tools/testing/selftests/bpf/progs/verifier_spill_fill.c
+++ b/tools/testing/selftests/bpf/progs/verifier_spill_fill.c
@@ -1244,4 +1244,22 @@ __naked void old_stack_misc_vs_cur_ctx_ptr(void)
 	: __clobber_all);
 }
 
+SEC("socket")
+__description("stack_noperfmon: reject read of invalid slots")
+__success
+__caps_unpriv(CAP_BPF)
+__failure_unpriv __msg_unpriv("invalid read from stack off -8+1 size 8")
+__naked void stack_noperfmon_reject_invalid_read(void)
+{
+	asm volatile ("					\
+	r2 = 1;						\
+	r6 = r10;					\
+	r6 += -8;					\
+	*(u8 *)(r6 + 0) = r2;				\
+	r2 = *(u64 *)(r6 + 0);				\
+	r0 = 0;						\
+	exit;						\
+"	::: __clobber_all);
+}
+
 char _license[] SEC("license") = "GPL";
-- 
2.43.5

[PATCH bpf v4 5/5] selftests/bpf: Add test for narrow spill into 64-bit spilled scalar

[Kumar Kartikeya Dwivedi]

Add a test case to verify that without CAP_PERFMON, the test now
succeeds instead of failing due to a verification error.

Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 .../selftests/bpf/progs/verifier_spill_fill.c   | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/tools/testing/selftests/bpf/progs/verifier_spill_fill.c b/tools/testing/selftests/bpf/progs/verifier_spill_fill.c
index bab6d789ba00..1e5a511e8494 100644
--- a/tools/testing/selftests/bpf/progs/verifier_spill_fill.c
+++ b/tools/testing/selftests/bpf/progs/verifier_spill_fill.c
@@ -1262,4 +1262,21 @@ __naked void stack_noperfmon_reject_invalid_read(void)
 "	::: __clobber_all);
 }
 
+SEC("socket")
+__description("stack_noperfmon: narrow spill onto 64-bit scalar spilled slots")
+__success
+__caps_unpriv(CAP_BPF)
+__success_unpriv
+__naked void stack_noperfmon_spill_32bit_onto_64bit_slot(void)
+{
+	asm volatile("					\
+	r0 = 0;						\
+	*(u64 *)(r10 - 8) = r0;				\
+	*(u32 *)(r10 - 8) = r0;				\
+	exit;						\
+"	:
+	:
+	: __clobber_all);
+}
+
 char _license[] SEC("license") = "GPL";
-- 
2.43.5

Re: [PATCH bpf v4 0/5] Fixes for stack with allow_ptr_leaks

[patchwork-bot+netdevbpf]

Hello:

This series was applied to bpf/bpf.git (master)
by Alexei Starovoitov <ast@kernel.org>:

On Tue,  3 Dec 2024 20:47:52 -0800 you wrote:
> Two fixes for usability/correctness gaps when interacting with the stack
> without CAP_PERFMON (i.e. with allow_ptr_leaks = false). See the commits
> for details. I've verified that the tests fail when run without the fixes.
> 
> Changelog:
> ----------
> v3 -> v4
> v3: https://lore.kernel.org/bpf/20241202083814.1888784-1-memxor@gmail.com
> 
> [...]

Here is the summary with links:
  - [bpf,v4,1/5] bpf: Don't mark STACK_INVALID as STACK_MISC in mark_stack_slot_misc
    https://git.kernel.org/bpf/bpf/c/69772f509e08
  - [bpf,v4,2/5] bpf: Fix narrow scalar spill onto 64-bit spilled scalar slots
    https://git.kernel.org/bpf/bpf/c/b0e66977dc07
  - [bpf,v4,3/5] selftests/bpf: Introduce __caps_unpriv annotation for tests
    https://git.kernel.org/bpf/bpf/c/adfdd9c68566
  - [bpf,v4,4/5] selftests/bpf: Add test for reading from STACK_INVALID slots
    https://git.kernel.org/bpf/bpf/c/f513c3635078
  - [bpf,v4,5/5] selftests/bpf: Add test for narrow spill into 64-bit spilled scalar
    https://git.kernel.org/bpf/bpf/c/19b6dbc006ec

You are awesome, thank you!
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