[PATCH] libbpf: fix UAF in strset__add_str()

[Carlos Llamas <cmllamas@google.com>]

strset_add_str_mem() might reallocate the strset data buffer in order to
accommodate the provided string 's'. However, if 's' points to a string
already present in the buffer, it becomes dangling after the realloc.
This leads to a use-after-free when attempting to memcpy() the string
into the new buffer.

One scenario that triggers this problematic path is when resolve_btfids
attempts to patch kfunc prototypes using existing BTF parameter names:

 | resolve_btfids: function bpf_list_push_back_impl already exists in BTF
 | Segmentation fault (core dumped)

Compiling resolve_btfids with fsanitize=address generates a detailed
report of the UAF:

 | =================================================================
 | ERROR: AddressSanitizer: heap-use-after-free on address 0x7f4c4a500bd4
 | ==1507892==ERROR: AddressSanitizer: heap-use-after-free on address 0x7f4c4a500bd4 at pc 0x55d25155a2a8 bp 0x7ffcef879060 sp 0x7ffcef878818
 | READ of size 5 at 0x7f4c4a500bd4 thread T0
 |     #0 0x55d25155a2a7 in memcpy (tools/bpf/resolve_btfids/resolve_btfids+0xcf2a7)
 |     #1 0x55d2515d708e in strset__add_str tools/lib/bpf/strset.c:162:2
 |     #2 0x55d2515c730b in btf__add_str tools/lib/bpf/btf.c:2109:8
 |     #3 0x55d2515c9020 in btf__add_func_param tools/lib/bpf/btf.c:3108:14
 |     #4 0x55d25159f0b5 in process_kfunc_with_implicit_args tools/bpf/resolve_btfids/main.c:1196:9
 |     #5 0x55d25159e004 in btf2btf tools/bpf/resolve_btfids/main.c:1229:9
 |     #6 0x55d25159cee7 in main tools/bpf/resolve_btfids/main.c:1535:6
 |     #7 0x7f4c78e29f76 in __libc_start_call_main csu/../sysdeps/nptl/libc_start_call_main.h:58:16
 |     #8 0x7f4c78e2a026 in __libc_start_main csu/../csu/libc-start.c:360:3
 |     #9 0x55d2514bb860 in _start (tools/bpf/resolve_btfids/resolve_btfids+0x30860)
 |
 | 0x7f4c4a500bd4 is located 13268 bytes inside of 2829000-byte region [0x7f4c4a4fd800,0x7f4c4a7b02c8)
 | freed by thread T0 here:
 |     #0 0x55d25155b700 in realloc (tools/bpf/resolve_btfids/resolve_btfids+0xd0700)
 |     #1 0x55d2515c426c in libbpf_reallocarray tools/lib/bpf/./libbpf_internal.h:220:9
 |     #2 0x55d2515c426c in libbpf_add_mem tools/lib/bpf/btf.c:224:13
 |
 | previously allocated by thread T0 here:
 |     #0 0x55d25155b2e3 in malloc (tools/bpf/resolve_btfids/resolve_btfids+0xd02e3)
 |     #1 0x55d2515d6e7d in strset__new tools/lib/bpf/strset.c:58:20

While resolve_btfids could be refactored to avoid this call path, let's
instead fix this issue at the source in strset__add_str() and avoid
similar scenarios. Let's simply check whether 's' is already within the
strset data buffer boundaries, and return the offset directly if so.

Fixes: 919d2b1dbb07 ("libbpf: Allow modification of BTF and add btf__add_str API")
Signed-off-by: Carlos Llamas <cmllamas@google.com>
---
 tools/lib/bpf/strset.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/tools/lib/bpf/strset.c b/tools/lib/bpf/strset.c
index 2464bcbd04e0..7d2b2784172e 100644
--- a/tools/lib/bpf/strset.c
+++ b/tools/lib/bpf/strset.c
@@ -141,10 +141,15 @@ int strset__find_str(struct strset *set, const char *s)
  */
 int strset__add_str(struct strset *set, const char *s)
 {
+	const char *strs = strset__data(set);
 	long old_off, new_off, len;
 	void *p;
 	int err;
 
+	/* Check whether 's' is already in the strset data buffer */
+	if (strs && s >= strs && s < strs + set->strs_data_len)
+		return s - strs;
+
 	/* Hashmap keys are always offsets within set->strs_data, so to even
 	 * look up some string from the "outside", we need to first append it
 	 * at the end, so that it can be addressed with an offset. Luckily,
-- 
2.54.0.563.g4f69b47b94-goog