On Mon, Jun 5, 2023 at 9:50 AM Krister Johansen kjlx@templeofstupid.com wrote:

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);

Instead of printk please do a volatile read of umyaddr->sa_family.

Please convert this commit log to a test in selftest/bpf/ and resubmit as two patches.

Also see bpf_testmod_return_ptr() and SEC("fexit/bpf_testmod_return_ptr") in progs/test_module_attach.c.

Probably easier to tweak that test for subprogs instead of adding your own SEC("fentry/__sys_bind") test and triggering bind() from user space.

 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@vger.kernel.org Fixes: 1c2a088a6626 ("bpf: x64: add JIT support for multi-function programs") Signed-off-by: Krister Johansen kjlx@templeofstupid.com

---

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++) {
  

() are redundant. !e is preferred over e == NULL

•                   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);
  

•           }
  

•   }
  

something odd here. We do bpf_prog_kallsyms_add(func[i]); for each subprog. So bpf_prog_ksym_find() in search_bpf_extables() should be finding ksym and extable of the subprog and not the main prog. The bug is probably elsewhere.

Once you respin with a selftest we can help debugging.