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Fix GH-16870: gmp_pow(64, 11) throws overflow exception #16884

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Fix GH-16870: gmp_pow(64, 11) throws overflow exception #16884

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d5ed93b
Fix GH-16870: gmp_pow(64, 11) throws overflow exception
cmb69 Nov 21, 2024
4acc1b2
Remove now unused variable
cmb69 Nov 21, 2024
9ce9a4e
Count bits instead of bytes
cmb69 Nov 24, 2024
333524e
Use `mpz_sizeinbase(gmpnum_base, 2) - 1`
cmb69 Nov 25, 2024
3998435
More thorough testing
cmb69 Nov 25, 2024
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Fix GH-16870: gmp_pow(64, 11) throws overflow exception 
The current guard to prevent FPEs is way too restrictive; `64 ** 11` is
a perfectly reasonable operation.  Instead, we now estimate the number
of bytes of the resulting GMP (assuming that numbers are stored base
256 encoded), and fail if that exceeds a given threshold.  The chosen
threshold is somewhat arbitrary.

We also ensure that we do not prematurely convert a given non int base
to an int to avoid overflow which could circumvent our early check.
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@cmb69
cmb69 committed Nov 25, 2024
commit d5ed93b1186d33f261b40189c7932863a4ee3c12
7 changes: 3 additions & 4 deletions ext/gmp/gmp.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1350,11 +1350,11 @@ ZEND_FUNCTION(gmp_pow)
RETURN_THROWS();
}

double powmax = log((double)ZEND_LONG_MAX);
double powmax = 2000;

if (Z_TYPE_P(base_arg) == IS_LONG && Z_LVAL_P(base_arg) >= 0) {
INIT_GMP_RETVAL(gmpnum_result);
if ((log(Z_LVAL_P(base_arg)) * exp) > powmax) {
if ((log(Z_LVAL_P(base_arg)) / log(256) * exp) > powmax) {
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Don't we just care if it can be "allocated reasonably" ?
Which is actually somewhat described in section 5.14 of the GMP manual (mpz_export)
image

Wonder if we shouldn't use a heuristic based on that?

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This is basically what is already happening in this patch. The difference is that I'm counting bytes, not bits. The logic is to estimate the number of bytes the result would have, and if this is greater than some threshold (powmax is badly named; should be rather max_size_in_bytes or so), trigger a value error.

mpz_sizeinbase() is only used in the else branch, because if the input is a zend_long, that wouldn't work. So I'm calculating log256(base_arg) * exp instead.

I'll update to counting bytes.

zend_value_error("base and exponent overflow");
RETURN_THROWS();
}
Expand All @@ -1364,8 +1364,7 @@ ZEND_FUNCTION(gmp_pow)
zend_ulong gmpnum;
FETCH_GMP_ZVAL(gmpnum_base, base_arg, temp_base, 1);
INIT_GMP_RETVAL(gmpnum_result);
gmpnum = mpz_get_ui(gmpnum_base);
if ((log(gmpnum) * exp) > powmax) {
if ((mpz_sizeinbase(gmpnum_base, 16) / 2.0 * exp) > powmax) {
FREE_GMP_TEMP(temp_base);
zend_value_error("base and exponent overflow");
RETURN_THROWS();
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16 changes: 16 additions & 0 deletions ext/gmp/tests/gh16870.phpt
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@@ -0,0 +1,16 @@
--TEST--
GH-16870 (gmp_pow(64, 11) throws overflow exception)
--EXTENSIONS--
gmp
--FILE--
<?php
var_dump((string) gmp_pow(64, 11));
try {
gmp_pow("18446744073709551616", 0x7fffffff);
} catch (ValueError $e) {
echo $e->getMessage(), "\n";
}
?>
--EXPECT--
string(20) "73786976294838206464"
base and exponent overflow