linux-kernel - Re: [PATCH] alpha: Fix personality flag propagation across an exec

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Message-ID: <bff3cfad8a87799101891b4f786c5104db9dab13.camel@physik.fu-berlin.de>
Date: Thu, 09 Jan 2025 09:46:47 +0100
From: John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>
To: Arnd Bergmann <arnd@...db.de>, Richard Henderson	
 <richard.henderson@...aro.org>, Matt Turner <mattst88@...il.com>, "Eric W.
 Biederman" <ebiederm@...ssion.com>, Kees Cook <kees@...nel.org>, "Paul E.
 McKenney" <paulmck@...nel.org>, 	linux-alpha@...r.kernel.org,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org
Cc: Michael Cree <mcree@...on.net.nz>, Sam James <sam@...too.org>, "Maciej
 W. Rozycki" <macro@...am.me.uk>, Geert Uytterhoeven <geert@...ux-m68k.org>,
 Michael Karcher	 <kernel@...rcher.dialup.fu-berlin.de>, Chris Hofstaedtler
 <zeha@...ian.org>, 	util-linux@...r.kernel.org, linux-mips@...r.kernel.org,
 	loongarch@...ts.linux.dev
Subject: Re: [PATCH] alpha: Fix personality flag propagation across an exec

Hi Arnd,

thanks a lot for your feedback!

On Thu, 2025-01-09 at 09:43 +0100, Arnd Bergmann wrote:
> On Thu, Jan 9, 2025, at 09:01, Arnd Bergmann wrote:
> > On Fri, Jan 3, 2025, at 15:01, John Paul Adrian Glaubitz wrote:
> > 
> > > 
> > >  #define SET_PERSONALITY(EX)					\
> > > -	set_personality(((EX).e_flags & EF_ALPHA_32BIT)		\
> > > -	   ? PER_LINUX_32BIT : PER_LINUX)
> > > +	set_personality((((EX).e_flags & EF_ALPHA_32BIT)	\
> > > +	   ? PER_LINUX_32BIT : PER_LINUX) | (current->personality & (~PER_MASK)))
> > 
> > This looks wrong to me: since ADDR_LIMIT_32BIT is not part of
> > PER_MASK, executing a regular binary from a taso binary no longer
> > reverts back to the entire 64-bit address space.
> > 
> > It seems that the behavior on most other architectures changed in 2012
> > commit 16f3e95b3209 ("cross-arch: don't corrupt personality flags upon
> > exec()").
> > 
> > At the time, the same bug existed on mips, parisc and tile, but those
> > got fixed quickly.
> 
> Correction: from what I can tell, mips still has the bug (and now
> also loongarch), it's just in SET_PERSONALITY2() now instead of
> SET_PERSONALITY():
> 
>         current->personality &= ~READ_IMPLIES_EXEC;
>         ...
>         p = personality(current->personality);                          \
>         if (p != PER_LINUX32 && p != PER_LINUX)                         \
>                 set_personality(PER_LINUX);                             \
> 
> personality() only returns the lower 8 bits (execution domain),
> so if any of them are set (BSD/HPUX/IRIX32/IRIX64/...), both
> the upper and the lower bits are cleared, otherwise neither
> of them are.
> 
> The behavior on the other architectures is that we clear the
> lower bits but keep the upper ones.

So, if I understand this correctly, we should just use PER_MASK on alpha
for 64-bit executables and allow the bits to be cleared for 32-bit binaries?

Adrian

-- 
 .''`.  John Paul Adrian Glaubitz
: :' :  Debian Developer
`. `'   Physicist
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