Re: [PATCH] x86: uaccess: fix regression in unsafe_get_user

[Andy Lutomirski]

> On Feb 16, 2019, at 12:18 PM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> 
>> On Sat, 16 Feb 2019, Jann Horn wrote:
>>> On Sat, Feb 16, 2019 at 12:59 AM <baloo@xxxxxxxxx> wrote:
>>> When extracting an initramfs, a filename may be near an allocation boundary.
>>> Should that happen, strncopy_from_user will invoke unsafe_get_user which
>>> may cross the allocation boundary. Should that happen, unsafe_get_user will
>>> trigger a page fault, and strncopy_from_user would then bailout to
>>> byte_at_a_time behavior.
>>> 
>>> unsafe_get_user is unsafe by nature, and rely on pagefault to detect boundaries.
>>> After 9da3f2b74054 ("x86/fault: BUG() when uaccess helpers fault on kernel addresses")
>>> it may no longer rely on pagefault as the new page fault handler would
>>> trigger a BUG().
>>> 
>>> This commit allows unsafe_get_user to explicitly trigger pagefaults and
>>> handle them directly with the error target label.
>> 
>> Oof. So basically the init code is full of things that just call
>> syscalls instead of using VFS functions (which don't actually exist
>> for everything), and the VFS syscalls use getname_flags(), which uses
>> strncpy_from_user(), which can access out-of-bounds pages on
>> architectures that set CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS, and
>> that in summary means that all the init code is potentially prone to
>> tripping over this?
> 
> Not all init code. It should be only the initramfs decompression.
> 
>> I don't particularly like this approach to fixing it, but I also don't
>> have any better ideas, so I guess unless someone else has a bright
>> idea, this patch might have to go in.
> 
> So we know that this happens in the context of decompress() which calls
> flush_buffer() for every chunk. flush_buffer() gets the start_address and
> the length. We also know that the fault can only happen within:
> 
>    start_address <= fault_address < start_address + length + 8;
> 
> So something like the untested workaround below should cover the initramfs
> oddity and avoid to weaken the protection for all other cases.

What is the actual problem?  Weâre not actually demand-faulting this data, are we?  Are we just overrunning the buffer because the from_user helpers are too clever?  Can we fix it for real by having the fancy helpers do *aligned* loads so that they donât overrun the buffer?  Heck, this might be faster, too.