possible UAF in eval_else when fuzzing aug_preview

[Anza2001]

Hello, I found a UAF when running the following fuzz driver in OSS-Fuzz environment.

#include "config.h"
#include "augeas.h"
#include "internal.h"
#include "memory.h"
#include "syntax.h"
#include "transform.h"
#include "errcode.h"

extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
    if (size < 1) {
        return 0; // Not enough data to work with
    }

    // Initialize augeas with default parameters
    struct augeas *aug = aug_init(NULL, NULL, 0);
    if (!aug) {
        return 0; // Failed to initialize augeas
    }

    // Create a null-terminated string from the input data
    char *input_str = (char *)malloc(size + 1);
    if (!input_str) {
        aug_close(aug);
        return 0; // Memory allocation failed
    }
    memcpy(input_str, data, size);
    input_str[size] = '\0';

    // Prepare the output parameter
    char *output_str = NULL;

    // Fuzz the aug_preview function
    int result = aug_preview(aug, input_str, &output_str);

    // Clean up
    free(input_str);
    if (output_str) {
        free(output_str);
    }
    aug_close(aug);

    return 0; // Always return 0
}

Here is the ASAN log:

==12==ERROR: AddressSanitizer: heap-use-after-free on address 0x511000011c68 at pc 0x556cccb9342d bp 0x7ffc389568c0 sp 0x7ffc389568b8
READ of size 8 at 0x511000011c68 thread T0
SCARINESS: 51 (8-byte-read-heap-use-after-free)
    #0 0x556cccb9342c in eval_else /src/augeas/src/pathx.c:1014:16
    #1 0x556cccb89240 in eval_binary /src/augeas/src/pathx.c:1209:9
    #2 0x556cccb89240 in eval_expr /src/augeas/src/pathx.c:1499:9
    #3 0x556cccb7352f in pathx_eval /src/augeas/src/pathx.c:2869:5
    #4 0x556cccb7352f in pathx_first /src/augeas/src/pathx.c:2889:27
    #5 0x556cccb6cd54 in aug_preview /src/augeas/src/augeas.c:1999:12
    #6 0x556cccb58a95 in LLVMFuzzerTestOneInput /src/augeas/augeas_aug_preview_fuzzer.cc:33:18
    #7 0x556ccca0d4d0 in fuzzer::Fuzzer::ExecuteCallback(unsigned char const*, unsigned long) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:614:13
    #8 0x556ccca0ccf5 in fuzzer::Fuzzer::RunOne(unsigned char const*, unsigned long, bool, fuzzer::InputInfo*, bool, bool*) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:516:7
    #9 0x556ccca0e4d5 in fuzzer::Fuzzer::MutateAndTestOne() /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:760:19
    #10 0x556ccca0f265 in fuzzer::Fuzzer::Loop(std::__Fuzzer::vector<fuzzer::SizedFile, std::__Fuzzer::allocator<fuzzer::SizedFile>>&) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:905:5
    #11 0x556ccc9fe0ab in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char const*, unsigned long)) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerDriver.cpp:914:6
    #12 0x556ccca29482 in main /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerMain.cpp:20:10
    #13 0x7f1f90b0a082 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x24082) (BuildId: 0323ab4806bee6f846d9ad4bccfc29afdca49a58)
    #14 0x556ccc9f092d in _start (/out/augeas_aug_preview_fuzzer+0xb792d)

DEDUP_TOKEN: eval_else--eval_binary--eval_expr
0x511000011c68 is located 104 bytes inside of 256-byte region [0x511000011c00,0x511000011d00)
freed by thread T0 here:
    #0 0x556cccb1968c in realloc /src/llvm-project/compiler-rt/lib/asan/asan_malloc_linux.cpp:82:3
    #1 0x556cccb98cf4 in mem_realloc_n /src/augeas/src/memory.c:98:11
    #2 0x556cccb92808 in make_value /src/augeas/src/pathx.c:564:13
    #3 0x556cccb92808 in eval_else /src/augeas/src/pathx.c:1013:28
    #4 0x556cccb89240 in eval_binary /src/augeas/src/pathx.c:1209:9
    #5 0x556cccb89240 in eval_expr /src/augeas/src/pathx.c:1499:9
    #6 0x556cccb7352f in pathx_eval /src/augeas/src/pathx.c:2869:5
    #7 0x556cccb7352f in pathx_first /src/augeas/src/pathx.c:2889:27
    #8 0x556cccb6cd54 in aug_preview /src/augeas/src/augeas.c:1999:12
    #9 0x556cccb58a95 in LLVMFuzzerTestOneInput /src/augeas/augeas_aug_preview_fuzzer.cc:33:18
    #10 0x556ccca0d4d0 in fuzzer::Fuzzer::ExecuteCallback(unsigned char const*, unsigned long) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:614:13
    #11 0x556ccca0ccf5 in fuzzer::Fuzzer::RunOne(unsigned char const*, unsigned long, bool, fuzzer::InputInfo*, bool, bool*) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:516:7
    #12 0x556ccca0e4d5 in fuzzer::Fuzzer::MutateAndTestOne() /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:760:19
    #13 0x556ccca0f265 in fuzzer::Fuzzer::Loop(std::__Fuzzer::vector<fuzzer::SizedFile, std::__Fuzzer::allocator<fuzzer::SizedFile>>&) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:905:5
    #14 0x556ccc9fe0ab in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char const*, unsigned long)) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerDriver.cpp:914:6
    #15 0x556ccca29482 in main /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerMain.cpp:20:10
    #16 0x7f1f90b0a082 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x24082) (BuildId: 0323ab4806bee6f846d9ad4bccfc29afdca49a58)

DEDUP_TOKEN: __interceptor_realloc--mem_realloc_n--make_value
previously allocated by thread T0 here:
    #0 0x556cccb1968c in realloc /src/llvm-project/compiler-rt/lib/asan/asan_malloc_linux.cpp:82:3
    #1 0x556cccb98cf4 in mem_realloc_n /src/augeas/src/memory.c:98:11
    #2 0x556cccb8150d in make_value /src/augeas/src/pathx.c:564:13
    #3 0x556cccb88442 in eval_filter /src/augeas/src/pathx.c:1457:24
    #4 0x556cccb88442 in eval_expr /src/augeas/src/pathx.c:1496:9
    #5 0x556cccb88137 in eval_binary /src/augeas/src/pathx.c:1169:5
    #6 0x556cccb88137 in eval_expr /src/augeas/src/pathx.c:1499:9
    #7 0x556cccb8803c in eval_binary /src/augeas/src/pathx.c:1164:5
    #8 0x556cccb8803c in eval_expr /src/augeas/src/pathx.c:1499:9
    #9 0x556cccb8803c in eval_binary /src/augeas/src/pathx.c:1164:5
    #10 0x556cccb8803c in eval_expr /src/augeas/src/pathx.c:1499:9
    #11 0x556cccb8803c in eval_binary /src/augeas/src/pathx.c:1164:5
    #12 0x556cccb8803c in eval_expr /src/augeas/src/pathx.c:1499:9
    #13 0x556cccb88137 in eval_binary /src/augeas/src/pathx.c:1169:5
    #14 0x556cccb88137 in eval_expr /src/augeas/src/pathx.c:1499:9
    #15 0x556cccb7352f in pathx_eval /src/augeas/src/pathx.c:2869:5
    #16 0x556cccb7352f in pathx_first /src/augeas/src/pathx.c:2889:27
    #17 0x556cccb6cd54 in aug_preview /src/augeas/src/augeas.c:1999:12
    #18 0x556cccb58a95 in LLVMFuzzerTestOneInput /src/augeas/augeas_aug_preview_fuzzer.cc:33:18
    #19 0x556ccca0d4d0 in fuzzer::Fuzzer::ExecuteCallback(unsigned char const*, unsigned long) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:614:13
    #20 0x556ccca0ccf5 in fuzzer::Fuzzer::RunOne(unsigned char const*, unsigned long, bool, fuzzer::InputInfo*, bool, bool*) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:516:7
    #21 0x556ccca0e4d5 in fuzzer::Fuzzer::MutateAndTestOne() /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:760:19
    #22 0x556ccca0f265 in fuzzer::Fuzzer::Loop(std::__Fuzzer::vector<fuzzer::SizedFile, std::__Fuzzer::allocator<fuzzer::SizedFile>>&) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:905:5
    #23 0x556ccc9fe0ab in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char const*, unsigned long)) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerDriver.cpp:914:6
    #24 0x556ccca29482 in main /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerMain.cpp:20:10
    #25 0x7f1f90b0a082 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x24082) (BuildId: 0323ab4806bee6f846d9ad4bccfc29afdca49a58)

DEDUP_TOKEN: __interceptor_realloc--mem_realloc_n--make_value
SUMMARY: AddressSanitizer: heap-use-after-free /src/augeas/src/pathx.c:1014:16 in eval_else
Shadow bytes around the buggy address:
  0x511000011980: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x511000011a00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x511000011a80: fa fa fa fa fa fa fa fa 00 00 00 00 00 00 00 00
  0x511000011b00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x511000011b80: 00 00 00 00 00 00 00 00 fa fa fa fa fa fa fa fa
=>0x511000011c00: fd fd fd fd fd fd fd fd fd fd fd fd fd[fd]fd fd
  0x511000011c80: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
  0x511000011d00: fa fa fa fa fa fa fa fa 00 00 00 00 00 00 00 00
  0x511000011d80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x511000011e00: 00 00 00 00 00 00 00 00 fa fa fa fa fa fa fa fa
  0x511000011e80: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07 
  Heap left redzone:       fa
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Container overflow:      fc
  Array cookie:            ac
  Intra object redzone:    bb
  ASan internal:           fe
  Left alloca redzone:     ca
  Right alloca redzone:    cb
==12==ABORTING
MS: 1 CopyPart-; base unit: df1540f40a67178e4c925cbf57d216fad0976944
0xa,0x20,0xa,0x66,0x69,0x6c,0x65,0x73,0x7c,0x4,0x20,0x7c,0x4,0x20,0x65,0x6c,0x73,0x65,0x20,0x86,0xa,0xa,0x7c,0xc2,0xc2,0xc2,0x2e,0x65,0x6c,0x73,0x65,0x20,0x86,0xa,0xa,0x7c,0xc2,0xc2,0xc2,0x2e,0xa,0x7c,0xa,0x86,0xa,0x7c,0x2e,
\012 \012files|\004 |\004 else \206\012\012|\302\302\302.else \206\012\012|\302\302\302.\012|\012\206\012|.
artifact_prefix='augeas_aug_preview_fuzzer_'; Test unit written to augeas_aug_preview_fuzzer_crash-96d565623a8d0be372edad03e38ef4bb824ddb74
Base64: CiAKZmlsZXN8BCB8BCBlbHNlIIYKCnzCwsIuZWxzZSCGCgp8wsLCLgp8CoYKfC4=