[Bug analyzer/109335] New: -Wanalyzer-malloc-leak false positives and false negatives

["colomar.6.4.3 at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109335

            Bug ID: 109335
           Summary: -Wanalyzer-malloc-leak false positives and false
                    negatives
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: analyzer
          Assignee: dmalcolm at gcc dot gnu.org
          Reporter: colomar.6.4.3 at gmail dot com
  Target Milestone: ---

Created attachment 54786
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=54786&action=edit
Preprocessed reproducer

Link:
<https://inbox.sourceware.org/gcc/45c0584d-b326-a975-7ebc-cef76e154530@gmail.com/T/#u>

With both GCC 12.2.0 (Debian), and GCC 13.0.1 20230315 (built from source),
I can reproduce these false positives.  I'm on Debian Sid with
libbsd-dev 0.11.7-4, and libc-dev 2.36-8.

The reproducer program is a small program that checks a password against a
hardcoded string, and conditionally prints "validated".  I wrote it
precisely to demonstrate how [[gnu::malloc(deallocator)]] can be used to
ensure that passwords are not leaked in memory, but I found out that it
fails to detect some conditions.

Here's the program (it uses agetpass(), as defined in the shadow project):

$ cat pass.c 
#include <err.h>
#include <errno.h>
#include <limits.h>
#include <readpassphrase.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>


#define PASS_MAX  BUFSIZ - 1


#define MALLOCARRAY(n, type)   ((type *) mallocarray(n, sizeof(type)))


[[gnu::malloc, gnu::malloc(free)]] void *mallocarray(size_t nmemb, size_t
size);
void erase_pass(char *pass);
[[gnu::malloc(erase_pass)]] char *agetpass(const char *prompt);


void
do_work(void)
{
        char  *pass;

        pass = agetpass("Enter your password: ");
        if (pass == NULL)
                err(EXIT_FAILURE, "agetpass");

        if (strcmp(pass, "secret") == 0)
                puts("validated");

        /* erase_pass() zeroes the memory (think of memset(3), or bzero(3))
           and then releases the memory to the system (think of free(3)).
           If you only call free(pass), then you release the memory to the
           system without zeroing it.  Remember it contains a password!
           We would be leaking a password into the system memory, which can
           later be assigned to a different process.

           So, we should call erase_pass() as soon as possible, but let's
           say we forgot, and just call free():
        */
#if defined(BUG_1)
        // We forgot to zero the memory.
        free(pass);
        // GCC correctly catches this as -Wmismatched-dealloc
#elif defined(BUG_2)
        // We zeroed, but forgot to free(3).
        bzero(pass, PASS_MAX + 2);
        // GCC misses this.
#elif defined(BUG_3)
        // We forgot both of them.
        // GCC also misses this.
#else
        erase_pass(pass);  // OK, but 2 false positives.
#endif
}


int
main(void)
{
        do_work();

        for (;;)
                sleep(1);
}


void *
mallocarray(size_t nmemb, size_t size)
{
        return reallocarray(NULL, nmemb, size);
}


char *
agetpass(const char *prompt)
{
        char    *pass;
        size_t  len;

        pass = MALLOCARRAY(PASS_MAX + 2, char);
        if (pass == NULL)
                return NULL;

        if (readpassphrase(prompt, pass, PASS_MAX + 2, RPP_REQUIRE_TTY) ==
NULL)
                goto fail;

        len = strlen(pass);
        if (len == PASS_MAX + 1) {
                errno = ENOBUFS;
                goto fail;
        }

        return pass;

fail:
        freezero(pass, PASS_MAX + 2);
        return NULL;
}


void
erase_pass(char *pass)
{
        freezero(pass, PASS_MAX + 2);
}



This shows the false positives:


$ cc -Wall -Wextra pass.c $(pkgconf --cflags --libs libbsd-overlay) -fanalyzer
-O3
pass.c: In function ‘agetpass’:
pass.c:84:12: warning: leak of ‘pass’ [CWE-401] [-Wanalyzer-malloc-leak]
   84 |         if (pass == NULL)
      |            ^
  ‘do_work’: events 1-3
    |
    |   22 | do_work(void)
    |      | ^~~~~~~
    |      | |
    |      | (1) entry to ‘do_work’
    |......
    |   26 |         pass = agetpass("Enter your password: ");
    |      |                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    |      |                |
    |      |                (2) allocated here
    |      |                (3) calling ‘agetpass’ from ‘do_work’
    |
    +--> ‘agetpass’: events 4-5
           |
           |   78 | agetpass(const char *prompt)
           |      | ^~~~~~~~
           |      | |
           |      | (4) entry to ‘agetpass’
           |......
           |   84 |         if (pass == NULL)
           |      |            ~
           |      |            |
           |      |            (5) ‘pass’ leaks here; was allocated at (2)
           |
pass.c:91:12: warning: leak of ‘pass’ [CWE-401] [-Wanalyzer-malloc-leak]
   91 |         if (len == PASS_MAX + 1) {
      |            ^
  ‘do_work’: events 1-3
    |
    |   22 | do_work(void)
    |      | ^~~~~~~
    |      | |
    |      | (1) entry to ‘do_work’
    |......
    |   26 |         pass = agetpass("Enter your password: ");
    |      |                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    |      |                |
    |      |                (2) allocated here
    |      |                (3) calling ‘agetpass’ from ‘do_work’
    |
    +--> ‘agetpass’: events 4-9
           |
           |   78 | agetpass(const char *prompt)
           |      | ^~~~~~~~
           |      | |
           |      | (4) entry to ‘agetpass’
           |......
           |   84 |         if (pass == NULL)
           |      |            ~
           |      |            |
           |      |            (5) following ‘false’ branch...
           |......
           |   87 |         if (readpassphrase(prompt, pass, PASS_MAX + 2,
RPP_REQUIRE_TTY) == NULL)
           |      |           
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
           |      |            ||
           |      |            |(6) ...to here
           |      |            (7) following ‘false’ branch...
           |......
           |   90 |         len = strlen(pass);
           |      |               ~~~~~~~~~~~~
           |      |               |
           |      |               (8) ...to here
           |   91 |         if (len == PASS_MAX + 1) {
           |      |            ~
           |      |            |
           |      |            (9) following ‘false’ branch (when ‘len !=
8192’)...
           |
         ‘agetpass’: event 10
           |
           |cc1:
           | (10): ...to here
           |
         ‘agetpass’: event 11
           |
           |   91 |         if (len == PASS_MAX + 1) {
           |      |            ^
           |      |            |
           |      |            (11) ‘pass’ leaks here; was allocated at (2)
           |


Maybe I'm missing something, but I don't think falanyzer is correct here.

For the false negatives you just need to compile the above with -DBUG_2 or
-DBUG_3.
I didn't copy here the results, because well, it's a negative, so it simply
shows
the same as the above (so the false negatives and false positives happen in the
same compilation, which is quite confusing).