[PATCH v2] smackfs: restrict bytes count in smackfs write functions

Tetsuo Handa penguin-kernel at i-love.sakura.ne.jp
Thu Jan 28 14:24:00 UTC 2021

On 2021/01/28 22:27, Sabyrzhan Tasbolatov wrote:
>> Doesn't this change break legitimate requests like
>>   char buffer[20000];
>>   memset(buffer, ' ', sizeof(buffer));
>>   memcpy(buffer + sizeof(buffer) - 10, "foo", 3);
>>   write(fd, buffer, sizeof(buffer));
>> ?
> It does, in this case. Then I need to patch another version with
> whitespace stripping before, after label. I just followed the same thing
> that I see in security/selinux/selinuxfs.c sel_write_enforce() etc.
> It has the same memdup_user_nul() and count >= PAGE_SIZE check prior to that.

Since sel_write_enforce() accepts string representation of an integer value, PAGE_SIZE is sufficient.
But since smk_write_onlycap() and smk_write_relabel_self() accept list of space-delimited words,
you need to prove why PAGE_SIZE does not break userspace in your patch.

Also, due to the "too small to fail" memory-allocation rule, memdup_user_nul() for
count < PAGE_SIZE * 8 bytes is "never fails with -ENOMEM unless SIGKILLed by the OOM
killer". Also, memdup_user_nul() for count >= PAGE_SIZE * (1 << MAX_ORDER) - 1 bytes is
"never succeeds". Thus, you can safely add

	if (count >= PAGE_SIZE * (1 << MAX_ORDER) - 1)
		return -EINVAL; // or -ENOMEM if you want compatibility

to smackfs write functions. But it is a strange requirement that the caller of
memdup_user_nul() has to be aware of upper limit in a way that we won't hit

	 * There are several places where we assume that the order value is sane
	 * so bail out early if the request is out of bound.
	if (unlikely(order >= MAX_ORDER)) {
		WARN_ON_ONCE(!(gfp_mask & __GFP_NOWARN));
		return NULL;

path. memdup_user_nul() side should do

	if (count >= PAGE_SIZE * (1 << MAX_ORDER) - 1)
		return -ENOMEM;

check and return -ENOMEM if memdup_user_nul() does not want to use __GFP_NOWARN.
I still believe that memdup_user_nul() side should be fixed.

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