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Heap buffer overflow in JS_ReadFunctionTag when cpool_count overflows function_size

Aperta
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#1,853 0 commenti 0 reazioni 0 assegnatari Vedi su GitHub

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Valutazione

Difficoltà
4/5
Tempo stimato
3-5 giorni
Idoneità per principianti
48/100
Tipo di issue
Bug
Chiarezza
Specificata chiaramente
Stato di attività
Attiva
Stack tecnologico
c
Ambito
backend, security

Direzione di ricerca

Start in quickjs.c at JS_ReadFunctionTag around the reported size computation and inspect free_function_bytecode, which releases the constant pool on failure. Check how bytecode counts and allocation sizes are validated elsewhere, then run the existing bytecode reader tests with the supplied reproducer. Done means malformed input is rejected without overflow or an out-of-bounds read.

Scritto dal modello di indicizzazione a partire dal testo della issue.

Descrizione

JS_ReadFunctionTag uses attacker-controlled cpool_count, local_count and byte_code_len directly in an int size computation, with no validation, then allocates that size and later walks cpool_count entries.

plaintext

// quickjs.c:39704
if (bc_get_leb128_int(s, &bc.cpool_count)) goto fail;
if (bc_get_leb128_int(s, &bc.byte_code_len)) goto fail;
if (bc_get_leb128_int(s, &local_count)) goto fail;

function_size = cpool_offset + bc.cpool_count * sizeof(*bc.cpool); // int overflow
function_size += local_count * sizeof(*bc.vardefs); // int overflow
function_size += bc.byte_code_len; // int overflow
b = js_mallocz(ctx, function_size);
sizeof(JSValue) is 16, so cpool_count > 134,217,728 wraps the multiplication. cpool_count = 0x10000001 gives * 16 = 16: a 16-byte allocation, while b->cpool_count stays at 268 million.

On the error path free_function_bytecode() then walks the full range:

plaintext

for (i = 0; i < b->cpool_count; i++)
JS_FreeValueRT(rt, b->cpool[i]); // past the end of the block
Reproducer:

plaintext

#include <stdio.h>
#include "quickjs.h"
int main(void) {
unsigned char buf[1 << 20];
size_t n = fread(buf, 1, sizeof(buf), stdin);
if (!n) return 0;
JSRuntime *rt = JS_NewRuntime();
JSContext *ctx = JS_NewContext(rt);
JSValue v = JS_ReadObject(ctx, buf, n, JS_READ_OBJ_BYTECODE);
if (JS_IsException(v)) { JSValue e = JS_GetException(ctx); JS_FreeValue(ctx, e); }
JS_FreeValue(ctx, v);
JS_FreeContext(ctx); JS_FreeRuntime(rt);
return 0;
}
65-byte input:

plaintext

1d ff ff ff ff 01 01 13 30 30 30 30 30 30 30 30
30 30 30 30 30 30 30 30 30 30 16 30 0d 01 01 01
01 00 30 30 01 30 01 30 30 30 30 07 07 30 30 30
30 30 30 0c 30 30 30 30 30 30 30 30 30 30 30 30
30
ff ff ff ff at offset 1 is the documented UINT32_MAX checksum escape hatch, so the checksum does not need to be recomputed.

Result:

plaintext

==165748==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x7abb0bde0980
READ of size 4
#0 free_function_bytecode quickjs.c:37168:45
#1 free_gc_object quickjs.c:7104:9
#2 free_zero_refcount quickjs.c:7123:9
#3 js_free_value_rt quickjs.c:7169:21
#6 JS_ReadFunctionTag quickjs.c:39938:5
#7 JS_ReadObjectRec quickjs.c:40414:15
#8 js_map_read quickjs.c:54055:19
#12 JS_ReadObject quickjs.c:40617:12
Also reproduces as a plain SIGSEGV (exit 139) on a -O2 build with no sanitizers, so it is not an ASAN artifact.

Tested on commit 7a23d8e (0.17.0), x86-64 Linux.

Suggested fix: validate before the size computation and do the arithmetic in size_t with overflow checks:

plaintext

if (bc.cpool_count < 0 || bc.cpool_count > JS_MAX_CPOOL_COUNT ||
local_count < 0 || local_count > JS_MAX_LOCAL_COUNT ||
bc.byte_code_len < 0 || bc.byte_code_len > JS_MAX_BYTECODE_LEN)
goto fail;
and free only the cpool entries actually initialised (the buffer is js_mallocz'd, so track the filled count or clamp b->cpool_count before any goto fail that reaches the destructor).

I know bytecode is out of scope per SECURITY.md — filing as a plain bug report, not a security report.

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