libbpf: Fix realloc API handling in zero-sized edge cases

realloc() and reallocarray() can either return NULL or a special
non-NULL pointer, if their size argument is zero. This requires a bit
more care to handle NULL-as-valid-result situation differently from
NULL-as-error case. This has caused real issues before ([0]), and just
recently bit again in production when performing bpf_program__attach_usdt().

This patch fixes 4 places that do or potentially could suffer from this
mishandling of NULL, including the reported USDT-related one.

There are many other places where realloc()/reallocarray() is used and
NULL is always treated as an error value, but all those have guarantees
that their size is always non-zero, so those spot don't need any extra
handling.

  [0] d08ab82f59d5 ("libbpf: Fix double-free when linker processes empty sections")

Fixes: 999783c8bbda ("libbpf: Wire up spec management and other arch-independent USDT logic")
Fixes: b63b3c490eee ("libbpf: Add bpf_program__set_insns function")
Fixes: 697f104db8a6 ("libbpf: Support custom SEC() handlers")
Fixes: b12688267280 ("libbpf: Change the order of data and text relocations.")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20230711024150.1566433-1-andrii@kernel.org

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 78635fe..63311a7 100644 (file)
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -6161,7 +6161,11 @@ static int append_subprog_relos(struct bpf_program *main_prog, struct bpf_progra
        if (main_prog == subprog)
                return 0;
        relos = libbpf_reallocarray(main_prog->reloc_desc, new_cnt, sizeof(*relos));
-       if (!relos)
+       /* if new count is zero, reallocarray can return a valid NULL result;
+        * in this case the previous pointer will be freed, so we *have to*
+        * reassign old pointer to the new value (even if it's NULL)
+        */
+       if (!relos && new_cnt)
                return -ENOMEM;
        if (subprog->nr_reloc)
                memcpy(relos + main_prog->nr_reloc, subprog->reloc_desc,
@@ -8532,7 +8536,8 @@ int bpf_program__set_insns(struct bpf_program *prog,
                return -EBUSY;
 
        insns = libbpf_reallocarray(prog->insns, new_insn_cnt, sizeof(*insns));
-       if (!insns) {
+       /* NULL is a valid return from reallocarray if the new count is zero */
+       if (!insns && new_insn_cnt) {
                pr_warn("prog '%s': failed to realloc prog code\n", prog->name);
                return -ENOMEM;
        }
@@ -8841,7 +8846,11 @@ int libbpf_unregister_prog_handler(int handler_id)
 
        /* try to shrink the array, but it's ok if we couldn't */
        sec_defs = libbpf_reallocarray(custom_sec_defs, custom_sec_def_cnt, sizeof(*sec_defs));
-       if (sec_defs)
+       /* if new count is zero, reallocarray can return a valid NULL result;
+        * in this case the previous pointer will be freed, so we *have to*
+        * reassign old pointer to the new value (even if it's NULL)
+        */
+       if (sec_defs || custom_sec_def_cnt == 0)
                custom_sec_defs = sec_defs;
 
        return 0;

diff --git a/tools/lib/bpf/usdt.c b/tools/lib/bpf/usdt.c
index f1a1415..37455d0 100644 (file)
--- a/tools/lib/bpf/usdt.c
+++ b/tools/lib/bpf/usdt.c
@@ -852,8 +852,11 @@ static int bpf_link_usdt_detach(struct bpf_link *link)
                 * system is so exhausted on memory, it's the least of user's
                 * concerns, probably.
                 * So just do our best here to return those IDs to usdt_manager.
+                * Another edge case when we can legitimately get NULL is when
+                * new_cnt is zero, which can happen in some edge cases, so we
+                * need to be careful about that.
                 */
-               if (new_free_ids) {
+               if (new_free_ids || new_cnt == 0) {
                        memcpy(new_free_ids + man->free_spec_cnt, usdt_link->spec_ids,
                               usdt_link->spec_cnt * sizeof(*usdt_link->spec_ids));
                        man->free_spec_ids = new_free_ids;