libsepol: check for valid sensitivity before lookup

Check the sensitivity is valid and thus the lookup in the name array
`p_sens_val_to_name` is valid.

Found by oss-fuzz (#42729, #42730, #42735, #42741)

    ==54784==The signal is caused by a READ memory access.
        #0 0x5a10f3 in mls_semantic_level_expand ./selinux/libsepol/src/expand.c:934:11
        #1 0x53839e in policydb_user_cache ./selinux/libsepol/src/policydb.c:972:7
        #2 0x5c6325 in hashtab_map ./selinux/libsepol/src/hashtab.c:236:10
        #3 0x5392e9 in policydb_index_others ./selinux/libsepol/src/policydb.c:1274:6
        #4 0x53f90a in policydb_read ./selinux/libsepol/src/policydb.c:4496:6
        #5 0x50c679 in LLVMFuzzerTestOneInput ./selinux/libsepol/fuzz/binpolicy-fuzzer.c:35:6
        #6 0x4409e3 in fuzzer::Fuzzer::ExecuteCallback(unsigned char const*, unsigned long) (./selinux/out/binpolicy-fuzzer+0x4409e3)
        #7 0x4295bf in fuzzer::RunOneTest(fuzzer::Fuzzer*, char const*, unsigned long) (./selinux/out/binpolicy-fuzzer+0x4295bf)
        #8 0x42f850 in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char const*, unsigned long)) (./selinux/out/binpolicy-fuzzer+0x42f850)
        #9 0x45b6d2 in main (./selinux/out/binpolicy-fuzzer+0x45b6d2)
        #10 0x7f059fcd71c9 in __libc_start_call_main csu/../sysdeps/nptl/libc_start_call_main.h:58:16
        #11 0x7f059fcd7277 in __libc_start_main csu/../csu/libc-start.c:409:3
        #12 0x423900 in _start (./out/binpolicy-fuzzer+0x423900)

Signed-off-by: Christian Göttsche <cgzones@googlemail.com>

libsepol/src/expand.c

@@ -929,6 +929,10 @@ int mls_semantic_level_expand(mls_semantic_level_t * sl, mls_level_t * l,
 	if (!sl->sens)
 		return 0;
 
+	/* Invalid sensitivity */
+	if (sl->sens > p->p_levels.nprim || !p->p_sens_val_to_name[sl->sens - 1])
+		return -1;
+
 	l->sens = sl->sens;
 	levdatum = (level_datum_t *) hashtab_search(p->p_levels.table,
 						    p->p_sens_val_to_name[l->sens - 1]);