Re: [PATCH] fs/read_write.c: Fix a broken signed integer overflow check.

[Ari Sundholm <ari@tuxera.com>]

Hello, Al,

On 2/7/22 16:58, Al Viro wrote:
> On Mon, Feb 07, 2022 at 02:07:11PM +0200, Ari Sundholm wrote:
>> The function generic_copy_file_checks() checks that the ends of the
>> input and output file ranges do not overflow. Unfortunately, there is
>> an issue with the check itself.
>>
>> Due to the integer promotion rules in C, the expressions
>> (pos_in + count) and (pos_out + count) have an unsigned type because
>> the count variable has the type uint64_t. Thus, in many cases where we
>> should detect signed integer overflow to have occurred (and thus one or
>> more of the ranges being invalid), the expressions will instead be
>> interpreted as large unsigned integers. This means the check is broken.
> 
> I must be slow this morning, but... which values of pos_in and count are
> caught by your check, but not by the original?
> 

Thank you for your response and questions.

Assuming an x86-64 target platform, please consider:

loff_t pos_out = 0x7FFFFFFFFFFEFFFFLL;
and
uint64_t count = 65537;

The type of the expression (pos_out + count) is a 64-bit unsigned type, 
by C's integer promotion rules. Its value is 0x8000000000000000ULL, that 
is, bit 63 is set.

The comparison (pos_out + count) < pos_out, again due to C's integer 
promotion rules, is unsigned. Thus, the comparison, in this case, is 
equivalent to:

0x8000000000000000ULL < 0x7FFFFFFFFFFEFFFFULL,

which is false. Please note that the LHS is not expressible as a 
positive integer of type loff_t. With larger values for count, the 
problem should become quite obvious, as some the offsets within the file 
would not be expressible as positive integers of type loff_t. But I 
digress. As we can see above, the overflow is missed.

With the LHS explicitly cast to loff_t, the comparison is equivalent to:

0x8000000000000000LL < 0x7FFFFFFFFFFEFFFFLL,

which is true, as the LHS is negative.

This has also been verified in practice, and was detected when running 
tests on special cases of the copy_file_range syscall on different 
filesystems.

>> -	if (pos_in + count < pos_in || pos_out + count < pos_out)
>> +	if ((loff_t)(pos_in + count) < pos_in ||
>> +			(loff_t)(pos_out + count) < pos_out)
> 
> Example, please.  Why do you need that comparison to be signed?

Please see the above.

I also created a small test program one can try on Compiler Explorer: 
https://godbolt.org/z/e76rb3Ec9

Please let me know if there are any further concerns.

Best regards,
Ari Sundholm
ari@tuxera.com