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Date:   Fri, 18 Feb 2022 08:29:59 +0100
From:   Arnd Bergmann <arnd@...nel.org>
To:     Christoph Hellwig <hch@....de>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Linux API <linux-api@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Russell King - ARM Linux <linux@...linux.org.uk>,
        Will Deacon <will@...nel.org>, Guo Ren <guoren@...nel.org>,
        Brian Cain <bcain@...eaurora.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Michal Simek <monstr@...str.eu>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Nick Hu <nickhu@...estech.com>,
        Greentime Hu <green.hu@...il.com>,
        Dinh Nguyen <dinguyen@...nel.org>,
        Stafford Horne <shorne@...il.com>,
        Helge Deller <deller@....de>,
        Michael Ellerman <mpe@...erman.id.au>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Rich Felker <dalias@...c.org>,
        David Miller <davem@...emloft.net>,
        Richard Weinberger <richard@....at>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Max Filippov <jcmvbkbc@...il.com>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        alpha <linux-alpha@...r.kernel.org>,
        "open list:SYNOPSYS ARC ARCHITECTURE" 
        <linux-snps-arc@...ts.infradead.org>, linux-csky@...r.kernel.org,
        "open list:QUALCOMM HEXAGON..." <linux-hexagon@...r.kernel.org>,
        linux-ia64@...r.kernel.org,
        linux-m68k <linux-m68k@...ts.linux-m68k.org>,
        "open list:BROADCOM NVRAM DRIVER" <linux-mips@...r.kernel.org>,
        Openrisc <openrisc@...ts.librecores.org>,
        Parisc List <linux-parisc@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
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        linux-s390 <linux-s390@...r.kernel.org>,
        Linux-sh list <linux-sh@...r.kernel.org>,
        sparclinux <sparclinux@...r.kernel.org>,
        linux-um <linux-um@...ts.infradead.org>,
        "open list:TENSILICA XTENSA PORT (xtensa)" 
        <linux-xtensa@...ux-xtensa.org>,
        Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH v2 05/18] x86: remove __range_not_ok()

On Fri, Feb 18, 2022 at 7:28 AM Christoph Hellwig <hch@....de> wrote:
> On Wed, Feb 16, 2022 at 02:13:19PM +0100, Arnd Bergmann wrote:
> > --- a/arch/x86/events/core.c
> > +++ b/arch/x86/events/core.c
> > @@ -2794,7 +2794,7 @@ perf_callchain_kernel(struct perf_callchain_entry_ctx *entry, struct pt_regs *re
> >  static inline int
> >  valid_user_frame(const void __user *fp, unsigned long size)
> >  {
> > -     return (__range_not_ok(fp, size, TASK_SIZE) == 0);
> > +     return __access_ok(fp, size);
> >  }
>
> valid_user_frame just need to go away and the following __get_user calls
> replaced with normal get_user ones.

As I understand it, that would not work here because get_user() calls
access_ok() rather than __access_ok(), and on x86 that can not be
called in NMI context.

It is a bit odd that x86 is the only architecture that has this check,
but adding
it was clearly intentional, see 7c4788950ba5 ("x86/uaccess, sched/preempt:
Verify access_ok() context").

> > diff --git a/arch/x86/kernel/dumpstack.c b/arch/x86/kernel/dumpstack.c
> > index 53de044e5654..da534fb7b5c6 100644
> > --- a/arch/x86/kernel/dumpstack.c
> > +++ b/arch/x86/kernel/dumpstack.c
> > @@ -85,7 +85,7 @@ static int copy_code(struct pt_regs *regs, u8 *buf, unsigned long src,
> >        * Make sure userspace isn't trying to trick us into dumping kernel
> >        * memory by pointing the userspace instruction pointer at it.
> >        */
> > -     if (__chk_range_not_ok(src, nbytes, TASK_SIZE_MAX))
> > +     if (!__access_ok((void __user *)src, nbytes))
> >               return -EINVAL;
>
> This one is not needed at all as copy_from_user_nmi already checks the
> access range.

Ok, removing this.

> > diff --git a/arch/x86/kernel/stacktrace.c b/arch/x86/kernel/stacktrace.c
> > index 15b058eefc4e..ee117fcf46ed 100644
> > --- a/arch/x86/kernel/stacktrace.c
> > +++ b/arch/x86/kernel/stacktrace.c
> > @@ -90,7 +90,7 @@ copy_stack_frame(const struct stack_frame_user __user *fp,
> >  {
> >       int ret;
> >
> > -     if (__range_not_ok(fp, sizeof(*frame), TASK_SIZE))
> > +     if (!__access_ok(fp, sizeof(*frame)))
> >               return 0;
>
> Just switch the __get_user calls below to get_user instead.

Same as the first one, I think we can't do this in NMI context.

         Arnd

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