bpf: Augment raw_tp arguments with PTR_MAYBE_NULL

Arguments to a raw tracepoint are tagged as trusted, which carries the
semantics that the pointer will be non-NULL.  However, in certain cases,
a raw tracepoint argument may end up being NULL. More context about this
issue is available in [0].

Thus, there is a discrepancy between the reality, that raw_tp arguments can
actually be NULL, and the verifier's knowledge, that they are never NULL,
causing explicit NULL check branch to be dead code eliminated.

A previous attempt [1], i.e. the second fixed commit, was made to
simulate symbolic execution as if in most accesses, the argument is a
non-NULL raw_tp, except for conditional jumps.  This tried to suppress
branch prediction while preserving compatibility, but surfaced issues
with production programs that were difficult to solve without increasing
verifier complexity. A more complete discussion of issues and fixes is
available at [2].

Fix this by maintaining an explicit list of tracepoints where the
arguments are known to be NULL, and mark the positional arguments as
PTR_MAYBE_NULL. Additionally, capture the tracepoints where arguments
are known to be ERR_PTR, and mark these arguments as scalar values to
prevent potential dereference.

Each hex digit is used to encode NULL-ness (0x1) or ERR_PTR-ness (0x2),
shifted by the zero-indexed argument number x 4. This can be represented
as follows:
1st arg: 0x1
2nd arg: 0x10
3rd arg: 0x100
... and so on (likewise for ERR_PTR case).

In the future, an automated pass will be used to produce such a list, or
insert __nullable annotations automatically for tracepoints. Each
compilation unit will be analyzed and results will be collated to find
whether a tracepoint pointer is definitely not null, maybe null, or an
unknown state where verifier conservatively marks it PTR_MAYBE_NULL.
A proof of concept of this tool from Eduard is available at [3].

Note that in case we don't find a specification in the raw_tp_null_args
array and the tracepoint belongs to a kernel module, we will
conservatively mark the arguments as PTR_MAYBE_NULL. This is because
unlike for in-tree modules, out-of-tree module tracepoints may pass NULL
freely to the tracepoint. We don't protect against such tracepoints
passing ERR_PTR (which is uncommon anyway), lest we mark all such
arguments as SCALAR_VALUE.

While we are it, let's adjust the test raw_tp_null to not perform
dereference of the skb->mark, as that won't be allowed anymore, and make
it more robust by using inline assembly to test the dead code
elimination behavior, which should still stay the same.

  [0]: https://lore.kernel.org/bpf/ZrCZS6nisraEqehw@jlelli-thinkpadt14gen4.remote.csb
  [1]: https://lore.kernel.org/all/20241104171959.2938862-1-memxor@gmail.com
  [2]: https://lore.kernel.org/bpf/20241206161053.809580-1-memxor@gmail.com
  [3]: https://github.com/eddyz87/llvm-project/tree/nullness-for-tracepoint-params

Reported-by: Juri Lelli <juri.lelli@redhat.com> # original bug
Reported-by: Manu Bretelle <chantra@meta.com> # bugs in masking fix
Fixes: 3f00c5239344 ("bpf: Allow trusted pointers to be passed to KF_TRUSTED_ARGS kfuncs")
Fixes: cb4158ce8ec8 ("bpf: Mark raw_tp arguments with PTR_MAYBE_NULL")
Reviewed-by: Eduard Zingerman <eddyz87@gmail.com>
Co-developed-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20241213221929.3495062-3-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 9 insertions, 10 deletions

diff --git a/tools/testing/selftests/bpf/progs/raw_tp_null.c b/tools/testing/selftests/bpf/progs/raw_tp_null.c
index 457f34c151e3..5927054b6dd9 100644
--- a/tools/testing/selftests/bpf/progs/raw_tp_null.c
+++ b/tools/testing/selftests/bpf/progs/raw_tp_null.c
@@ -3,6 +3,7 @@
 
 #include <vmlinux.h>
 #include <bpf/bpf_tracing.h>
+#include "bpf_misc.h"
 
 char _license[] SEC("license") = "GPL";
 
@@ -17,16 +18,14 @@ int BPF_PROG(test_raw_tp_null, struct sk_buff *skb)
 	if (task->pid != tid)
 		return 0;
 
-	i = i + skb->mark + 1;
-	/* The compiler may move the NULL check before this deref, which causes
-	 * the load to fail as deref of scalar. Prevent that by using a barrier.
+	/* If dead code elimination kicks in, the increment +=2 will be
+	 * removed. For raw_tp programs attaching to tracepoints in kernel
+	 * modules, we mark input arguments as PTR_MAYBE_NULL, so branch
+	 * prediction should never kick in.
 	 */
-	barrier();
-	/* If dead code elimination kicks in, the increment below will
-	 * be removed. For raw_tp programs, we mark input arguments as
-	 * PTR_MAYBE_NULL, so branch prediction should never kick in.
-	 */
-	if (!skb)
-		i += 2;
+	asm volatile ("%[i] += 1; if %[ctx] != 0 goto +1; %[i] += 2;"
+			: [i]"+r"(i)
+			: [ctx]"r"(skb)
+			: "memory");
 	return 0;
 }