Re: [PATCH v3 20/23] fs: Allow copy_mount_options() to access user-space in a single pass

From: Catalin Marinas <catalin.marinas@arm.com>
To: Dave Martin <Dave.Martin@arm.com>
Cc: linux-arm-kernel@lists.infradead.org, linux-arch@vger.kernel.org,
	Richard Earnshaw <Richard.Earnshaw@arm.com>,
	Szabolcs Nagy <szabolcs.nagy@arm.com>,
	Andrey Konovalov <andreyknvl@google.com>,
	Kevin Brodsky <kevin.brodsky@arm.com>,
	Peter Collingbourne <pcc@google.com>,
	linux-mm@kvack.org, Alexander Viro <viro@zeniv.linux.org.uk>,
	Vincenzo Frascino <vincenzo.frascino@arm.com>,
	Will Deacon <will@kernel.org>
Subject: Re: [PATCH v3 20/23] fs: Allow copy_mount_options() to access user-space in a single pass
Date: Wed, 29 Apr 2020 14:52:25 +0100	[thread overview]
Message-ID: <20200429135224.GB10651@gaia> (raw)
In-Reply-To: <20200429102650.GC30377@arm.com>

On Wed, Apr 29, 2020 at 11:26:51AM +0100, Dave P Martin wrote:
> On Tue, Apr 21, 2020 at 03:26:00PM +0100, Catalin Marinas wrote:
> > diff --git a/arch/arm64/include/asm/uaccess.h b/arch/arm64/include/asm/uaccess.h
> > index 32fc8061aa76..566da441eba2 100644
> > --- a/arch/arm64/include/asm/uaccess.h
> > +++ b/arch/arm64/include/asm/uaccess.h
> > @@ -416,6 +416,17 @@ extern unsigned long __must_check __arch_copy_in_user(void __user *to, const voi
> >  #define INLINE_COPY_TO_USER
> >  #define INLINE_COPY_FROM_USER
> >  
> > +static inline bool arch_has_exact_copy_from_user(unsigned long n)
> > +{
> > +	/*
> > +	 * copy_from_user() aligns the source pointer if the size is greater
> > +	 * than 15. Since all the loads are naturally aligned, they can only
> > +	 * fail on the first byte.
> > +	 */
> > +	return n > 15;
> > +}
> > +#define arch_has_exact_copy_from_user
> 
> Did you mean:
> 
> #define arch_has_exact_copy_from_user arch_has_exact_copy_from_user

Yes (and I shouldn't write patches late in the day).

> Mind you, if this expands to 1 I'd have expected copy_mount_options()
> not to compile, so I may be missing something.

I think arch_has_exact_copy_from_user() (with the braces) is looked up
in the function namespace, so the macro isn't expanded. So arguably the
patch is correct but pretty dodgy ;).

I scrapped this in my second attempt in reply to Kevin.

> > diff --git a/fs/namespace.c b/fs/namespace.c
> > index a28e4db075ed..8febc50dfc5d 100644
> > --- a/fs/namespace.c
> > +++ b/fs/namespace.c
> > @@ -3025,13 +3025,16 @@ void *copy_mount_options(const void __user * data)
> 
> [ Is this applying a band-aid to duct tape?
> 
> The fs presumably knows ahead of time whether it's expecting a string or
> a fixed-size blob for data, so I'd hope we could just DTRT rather than
> playing SEGV roulette here.
> 
> This might require more refactoring than makes sense for this series
> though. ]

That's possible but it means moving the copy from sys_mount() to the
specific places where it has additional information (the filesystems).
I'm not even sure it's guaranteed to be strings. If it is, we could just
replace all this with a strncpy_from_user().

-- 
Catalin