[PATCH bpf] bpf: search_bpf_extables should search subprogram extables

[Krister Johansen]

JIT'd bpf programs that have subprograms can have a postive value for
num_extentries but a NULL value for extable.  This is problematic if one of
these bpf programs encounters a fault during its execution.  The fault
handlers correctly identify that the faulting IP belongs to a bpf program.
However, performing a search_extable call on a NULL extable leads to a
second fault.

Fix up by refusing to search a NULL extable, and by checking the
subprograms' extables if the umbrella program has subprograms configured.

Once I realized what was going on, I was able to use the following bpf
program to get an oops from this failure:

   #include "vmlinux.h"
   #include <bpf/bpf_helpers.h>
   #include <bpf/bpf_tracing.h>
   
   char LICENSE[] SEC("license") = "Dual BSD/GPL";
   
   #define PATH_MAX 4096
   
   struct callback_ctx {
           u8 match;
   };
   
   struct filter_value {
           char prefix[PATH_MAX];
   };
   struct {
           __uint(type, BPF_MAP_TYPE_ARRAY);
           __uint(max_entries, 256);
           __type(key, int);
           __type(value, struct filter_value);
   } test_filter SEC(".maps");
   
   static __u64 test_filter_cb(struct bpf_map *map, __u32 *key,
                               struct filter_value *val,
                               struct callback_ctx *data)
   {
       return 1;
   }
   
   SEC("fentry/__sys_bind")
   int BPF_PROG(__sys_bind, int fd, struct sockaddr *umyaddr, int addrlen)
   {
     pid_t pid;
   
     struct callback_ctx cx = { .match = 0 };
     pid = bpf_get_current_pid_tgid() >> 32;
     bpf_for_each_map_elem(&test_filter, test_filter_cb, &cx, 0);
     bpf_printk("fentry: pid = %d, family = %llx\n", pid, umyaddr->sa_family);
     return 0;
   }

And then the following code to actually trigger a failure:

  #include <stdio.h>
  #include <stdlib.h>
  #include <unistd.h>
  #include <sys/socket.h>
  #include <netinet/in.h>
  #include <netinet/ip.h>
  
  int
  main(int argc, char *argv[])
  {
    int sfd, rc;
    struct sockaddr *sockptr = (struct sockaddr *)0x900000000000;
  
    sfd = socket(AF_INET, SOCK_STREAM, 0);
    if (sfd < 0) {
      perror("socket");
      exit(EXIT_FAILURE);
    }
  
    while (1) {
      rc = bind(sfd, (struct sockaddr *) sockptr, sizeof(struct sockaddr_in));
      if (rc < 0) {
        perror("bind");
        sleep(5);
      } else {
        break;
      }
    }
  
    return 0;
  }

I was able to validate that this problem does not occur when subprograms
are not in use, or when the direct pointer accesses are replaced with
bpf_probe_read calls.  I further validated that this did not break the
extable handling in existing bpf programs.  The same program caused no
failures when subprograms were removed, but the exception was still
injected.

Cc: stable@xxxxxxxxxxxxxxx
Fixes: 1c2a088a6626 ("bpf: x64: add JIT support for multi-function programs")
Signed-off-by: Krister Johansen <kjlx@xxxxxxxxxxxxxxxxxx>
---
 kernel/bpf/core.c | 22 ++++++++++++++++++++--
 1 file changed, 20 insertions(+), 2 deletions(-)

diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index 7421487422d4..0e12238e4340 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -736,15 +736,33 @@ const struct exception_table_entry *search_bpf_extables(unsigned long addr)
 {
 	const struct exception_table_entry *e = NULL;
 	struct bpf_prog *prog;
+	struct bpf_prog_aux *aux;
+	int i;
 
 	rcu_read_lock();
 	prog = bpf_prog_ksym_find(addr);
 	if (!prog)
 		goto out;
-	if (!prog->aux->num_exentries)
+	aux = prog->aux;
+	if (!aux->num_exentries)
 		goto out;
 
-	e = search_extable(prog->aux->extable, prog->aux->num_exentries, addr);
+	/* prog->aux->extable can be NULL if subprograms are in use. In that
+	 * case, check each sub-function's aux->extables to see if it has a
+	 * matching entry.
+	 */
+	if (aux->extable != NULL) {
+		e = search_extable(prog->aux->extable,
+		    prog->aux->num_exentries, addr);
+	} else {
+		for (i = 0; (i < aux->func_cnt) && (e == NULL); i++) {
+			if (!aux->func[i]->aux->num_exentries ||
+			    aux->func[i]->aux->extable == NULL)
+				continue;
+			e = search_extable(aux->func[i]->aux->extable,
+			    aux->func[i]->aux->num_exentries, addr);
+		}
+	}
 out:
 	rcu_read_unlock();
 	return e;
-- 
2.25.1