From: Kees Cook <keescook@chromium.org>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Alexey Dobriyan <adobriyan@gmail.com>,
linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org
Subject: Re: [PATCH] elf: align AT_RANDOM bytes
Date: Wed, 29 May 2019 16:00:23 -0700 [thread overview]
Message-ID: <201905291559.87E96F79@keescook> (raw)
In-Reply-To: <20190529152020.c9d0ed1c6194328f751fe0f9@linux-foundation.org>
On Wed, May 29, 2019 at 03:20:20PM -0700, Andrew Morton wrote:
> On Thu, 30 May 2019 00:37:08 +0300 Alexey Dobriyan <adobriyan@gmail.com> wrote:
>
> > AT_RANDOM content is always misaligned on x86_64:
> >
> > $ LD_SHOW_AUXV=1 /bin/true | grep AT_RANDOM
> > AT_RANDOM: 0x7fff02101019
> >
> > glibc copies first few bytes for stack protector stuff, aligned
> > access should be slightly faster.
>
> I just don't understand the implications of this. Is there
> (badly-behaved) userspace out there which makes assumptions about the
> current alignment?
>
> How much faster, anyway? How frequently is the AT_RANDOM record
> accessed?
>
> I often have questions such as these about your performance/space
> tweaks :(. Please try to address them as a matter of course when
> preparing changelogs?
>
> And let's Cc Kees, who wrote the thing.
>
> > --- a/fs/binfmt_elf.c
> > +++ b/fs/binfmt_elf.c
> > @@ -144,11 +144,15 @@ static int padzero(unsigned long elf_bss)
> > #define STACK_ALLOC(sp, len) ({ \
> > elf_addr_t __user *old_sp = (elf_addr_t __user *)sp; sp += len; \
> > old_sp; })
> > +#define STACK_ALIGN(sp, align) \
> > + ((typeof(sp))(((unsigned long)sp + (int)align - 1) & ~((int)align - 1)))
>
> I suspect plain old ALIGN() could be used here.
>
> > #else
> > #define STACK_ADD(sp, items) ((elf_addr_t __user *)(sp) - (items))
> > #define STACK_ROUND(sp, items) \
> > (((unsigned long) (sp - items)) &~ 15UL)
> > #define STACK_ALLOC(sp, len) ({ sp -= len ; sp; })
> > +#define STACK_ALIGN(sp, align) \
> > + ((typeof(sp))((unsigned long)sp & ~((int)align - 1)))
>
> And maybe there's a helper which does this, dunno.
>
> > #endif
> >
> > #ifndef ELF_BASE_PLATFORM
> > @@ -217,6 +221,12 @@ create_elf_tables(struct linux_binprm *bprm, struct elfhdr *exec,
> > return -EFAULT;
> > }
> >
> > + /*
> > + * glibc copies first bytes for stack protector purposes
> > + * which are misaligned on x86_64 because strlen("x86_64") + 1 == 7.
> > + */
> > + p = STACK_ALIGN(p, sizeof(long));
> > +
I have no objection to eating some bytes here. Though perhaps things could just
be reordered to leave all the aligned things together and put all the
strings later?
-Kees
> > /*
> > * Generate 16 random bytes for userspace PRNG seeding.
> > */
>
--
Kees Cook
next prev parent reply other threads:[~2019-05-29 23:00 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-05-29 21:37 [PATCH] elf: align AT_RANDOM bytes Alexey Dobriyan
2019-05-29 22:20 ` Andrew Morton
2019-05-29 23:00 ` Kees Cook [this message]
2019-05-30 6:47 ` Alexey Dobriyan
2019-05-30 6:44 ` Alexey Dobriyan
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