Re: [PATCH 1/2] x86: unify copy_from_user() size checking

From: Guenter Roeck
Date: Sat Oct 26 2013 - 12:05:45 EST


On 10/26/2013 03:31 AM, Ingo Molnar wrote:

* Jan Beulich <JBeulich@xxxxxxxx> wrote:

Commits 4a3127693001c61a21d1ce680db6340623f52e93 ("x86: Turn the
copy_from_user check into an (optional) compile time warning") and
63312b6a6faae3f2e5577f2b001e3b504f10a2aa ("x86: Add a Kconfig option to
turn the copy_from_user warnings into errors") touched only the 32-bit
variant of copy_from_user(), whereas the original commit
9f0cf4adb6aa0bfccf675c938124e68f7f06349d ("x86: Use
__builtin_object_size() to validate the buffer size for
copy_from_user()") also added the same code to the 64-bit one.

Further the earlier conversion from an inline WARN() to the call to
copy_from_user_overflow() went a little too far: When the number of
bytes to be copied is not a constant (e.g. [looking at 3.11] in
drivers/net/tun.c:__tun_chr_ioctl() or
drivers/pci/pcie/aer/aer_inject.c:aer_inject_write()), the compiler
will always have to keep the funtion call, and hence there will always
be a warning. By using __builtin_constant_p() we can avoid this.

And then this slightly extends the effect of
CONFIG_DEBUG_STRICT_USER_COPY_CHECKS in that apart from converting
warnings to errors in the constant size case, it retains the (possibly
wrong) warnings in the non-constant size case, such that if someone is
prepared to get a few false positives, (s)he'll be able to recover the
current behavior (except that these diagnostics now will never be
converted to errors).

Since the 32-bit variant (intentionally) didn't call might_fault(), the
unification results in this being called twice now. Adding a suitable
#ifdef would be the alternative if that's a problem.

Addressing that sanely would be nice.

I'd like to point out though that with __compiletime_object_size()
being restricted to gcc before 4.6, the whole construct is going to
become more and more pointless going forward. I would question
however that commit 2fb0815c9ee6b9ac50e15dd8360ec76d9fa46a2 ("gcc4:
disable __compiletime_object_size for GCC 4.6+") was really necessary,
and instead this should have been dealt with as is done here from the
beginning.

Can we now revert 2fb0815c9ee6?


I sure don't mind if there is a different and better solution.

Guenter

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