Add a verifier test that a BPF_LOAD_ACQ from a rdonly_untrusted_mem pointer
(PTR_TO_MEM | MEM_RDONLY | PTR_UNTRUSTED, obtained via bpf_rdonly_cast())
is rejected. Such a source requires BPF_PROBE_MEM fault protection which
is not applied to atomic loads; without the verifier fix the load is accepted
and would crash the kernel on a fault.

  # LDLIBS=-static PKG_CONFIG='pkg-config --static' ./vmtest.sh -- ./test_progs -t verifier_load_acquire
  [...]
  #621/1   verifier_load_acquire/load-acquire, 8-bit:OK
  #621/2   verifier_load_acquire/load-acquire, 8-bit @unpriv:OK
  #621/3   verifier_load_acquire/load-acquire, 16-bit:OK
  #621/4   verifier_load_acquire/load-acquire, 16-bit @unpriv:OK
  #621/5   verifier_load_acquire/load-acquire, 32-bit:OK
  #621/6   verifier_load_acquire/load-acquire, 32-bit @unpriv:OK
  #621/7   verifier_load_acquire/load-acquire, 64-bit:OK
  #621/8   verifier_load_acquire/load-acquire, 64-bit @unpriv:OK
  [...]
  #621/19  verifier_load_acquire/load-acquire from rdonly_untrusted_mem pointer:OK
  #621/20  verifier_load_acquire/load-acquire with invalid register R15:OK
  #621/21  verifier_load_acquire/load-acquire with invalid register R15 @unpriv:OK
  #621/22  verifier_load_acquire/load-acquire from pkt pointer:OK
  #621/23  verifier_load_acquire/load-acquire from flow_keys pointer:OK
  #621/24  verifier_load_acquire/load-acquire from sock pointer:OK
  #621     verifier_load_acquire:OK
  Summary: 1/24 PASSED, 0 SKIPPED, 0 FAILED

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
---
 .../bpf/progs/verifier_load_acquire.c         | 28 +++++++++++++++++++
 1 file changed, 28 insertions(+)

diff --git a/tools/testing/selftests/bpf/progs/verifier_load_acquire.c b/tools/testing/selftests/bpf/progs/verifier_load_acquire.c
index ae1dab1b0cbb..d17026d7480d 100644
--- a/tools/testing/selftests/bpf/progs/verifier_load_acquire.c
+++ b/tools/testing/selftests/bpf/progs/verifier_load_acquire.c
@@ -3,6 +3,7 @@
 
 #include <linux/bpf.h>
 #include <bpf/bpf_helpers.h>
+#include <bpf/bpf_core_read.h>
 #include "../../../include/linux/filter.h"
 #include "bpf_misc.h"
 
@@ -221,6 +222,33 @@ __naked void load_acquire_from_sock_pointer(void)
 	: __clobber_all);
 }
 
+SEC("socket")
+__description("load-acquire from rdonly_untrusted_mem pointer")
+__failure __msg("BPF_ATOMIC loads from R{{[0-9]+}} rdonly_untrusted_mem is not allowed")
+int load_acquire_from_rdonly_untrusted_mem(void *ctx)
+{
+	__u64 val = 0;
+	void *p;
+
+	/*
+	 * bpf_rdonly_cast(x, 0) yields PTR_TO_MEM | MEM_RDONLY | PTR_UNTRUSTED.
+	 * A regular BPF_LDX from it is rewritten to BPF_PROBE_MEM, but a
+	 * load-acquire is not, so it must be rejected, otherwise the JIT emits
+	 * a plain load with no exception table entry and a fault would crash
+	 * the kernel.
+	 */
+	p = bpf_rdonly_cast(&val, 0);
+	asm volatile (
+	"r1 = %[p];"
+	".8byte %[load_acquire_insn];" // r0 = load_acquire((u64 *)(r1 + 0));
+	:
+	: [p] "r" (p),
+	  __imm_insn(load_acquire_insn,
+		     BPF_ATOMIC_OP(BPF_DW, BPF_LOAD_ACQ, BPF_REG_0, BPF_REG_1, 0))
+	: "r0", "r1");
+	return 0;
+}
+
 SEC("socket")
 __description("load-acquire with invalid register R15")
 __failure __failure_unpriv __msg("R15 is invalid")
-- 
2.43.0