linux-kernel - Re: [PATCH v14 1/4] asm-generic,arm64: create task variant of access

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Date:   Wed, 29 Mar 2023 00:34:04 -0400
From:   Gregory Price <gregory.price@...verge.com>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     Gregory Price <gourry.memverge@...il.com>,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-arch@...r.kernel.org,
        oleg@...hat.com, avagin@...il.com, peterz@...radead.org,
        luto@...nel.org, krisman@...labora.com, tglx@...utronix.de,
        corbet@....net, shuah@...nel.org, arnd@...db.de, will@...nel.org,
        mark.rutland@....com, tongtiangen@...wei.com, robin.murphy@....com
Subject: Re: [PATCH v14 1/4] asm-generic,arm64: create task variant of
 access_ok

On Wed, Mar 29, 2023 at 05:22:49PM +0100, Catalin Marinas wrote:
> On Tue, Mar 28, 2023 at 12:48:08PM -0400, Gregory Price wrote:
> > diff --git a/arch/arm64/include/asm/uaccess.h b/arch/arm64/include/asm/uaccess.h
> > index 5c7b2f9d5913..1a51a54f264f 100644
> > --- a/arch/arm64/include/asm/uaccess.h
> > +++ b/arch/arm64/include/asm/uaccess.h
> > @@ -35,7 +35,9 @@ static inline int __access_ok(const void __user *ptr, unsigned long size);
> >   * This is equivalent to the following test:
> >   * (u65)addr + (u65)size <= (u65)TASK_SIZE_MAX
> >   */
> > -static inline int access_ok(const void __user *addr, unsigned long size)
> > +static inline int task_access_ok(struct task_struct *task,
> > +				 const void __user *addr,
> > +				 unsigned long size)
> >  {
> >  	/*
> >  	 * Asynchronous I/O running in a kernel thread does not have the
> > @@ -43,11 +45,18 @@ static inline int access_ok(const void __user *addr, unsigned long size)
> >  	 * the user address before checking.
> >  	 */
> >  	if (IS_ENABLED(CONFIG_ARM64_TAGGED_ADDR_ABI) &&
> > -	    (current->flags & PF_KTHREAD || test_thread_flag(TIF_TAGGED_ADDR)))
> > +	    (task->flags & PF_KTHREAD || test_ti_thread_flag(task, TIF_TAGGED_ADDR)))
> >  		addr = untagged_addr(addr);
> >  
> >  	return likely(__access_ok(addr, size));
> >  }
> > +
> > +static inline int access_ok(const void __user *addr, unsigned long size)
> > +{
> > +	return task_access_ok(current, addr, size);
> > +}
> > +
> > +#define task_access_ok task_access_ok
> 
> I'd not bother with this at all. In the generic code you can either do
> an __access_ok() check directly or just
> access_ok(untagged_addr(selector), ...) with a comment that address
> tagging of the ptraced task may not be enabled.
> 
> -- 
> Catalin

This was my original proposal, but the comment that lead to this patch
was the following:

"""
If this would be correct, then access_ok() on arm64 would
unconditionally untag the checked address, but it does not. Simply
because untagging is only valid if the task enabled pointer tagging. If
it didn't a tagged pointer is obviously invalid.

Why would ptrace make this suddenly valid?
"""

https://lore.kernel.org/all/87a605anvx.ffs@tglx/

I did not have a sufficient answer for this so I went down this path.

It does seem simpler to simply untag the address, however it didn't seem
like a good solution to simply leave an identified bad edge case.

with access_ok(untagged_addr(addr), ...) it breaks down like this:

(tracer,tracee) : result 

tag,tag     : untagged - (correct)
tag,untag   : untagged - incorrect as this would have been an impossible
              state to reach through the standard prctl interface.  Will
	      lead to a SIGSEGV in the tracee upon next syscall
untag,tag   : untagged - (correct)
untag,untag : no-op - (correct), tagged address will fail to set

Basically if the tracer is a tagged process while the tracee is not, it
would become possible to set the tracee's selector to a tagged pointer.

It's beyond me to say whether or not this situation is "ok" and "the
user's fault", but it does feel like an addressable problem.

Thoughts?
~Gregory