Re: [PATCH v2] MIPS: Add nonxstack=on|off kernel parameter

[James Hogan]

On Thu, Nov 30, 2017 at 09:34:15AM +0000, Miodrag Dinic wrote:
> Hi David,
> 
> Sorry for a late response, please find answers in-lined:
> 
> > > If this parameter is omitted, kernel behavior remains the same as it
> > > was before this patch is applied.
> > 
> > Do other architectures have a similar hack?
> > 
> > If arm{,64} and x86 don't need this, what would make MIPS so special
> > that we have to carry this around?
> 
> Yes, there are similar workarounds. Just a couple lines above
> nonxstack description in the documentation there are :
> 	noexec		[IA-64]
> 
> 	noexec		[X86]
> 			On X86-32 available only on PAE configured kernels.
> 			noexec=on: enable non-executable mappings (default)
> 			noexec=off: disable non-executable mappings
> ...
> 
> 	noexec32	[X86-64]
> 			This affects only 32-bit executables.
> 			noexec32=on: enable non-executable mappings (default)
> 				read doesn't imply executable mappings
> 			noexec32=off: disable non-executable mappings
> 				read implies executable mappings
> 
> > > 
> > > This functionality is convenient during debugging and is especially
> > > useful for Android development where non-exec stack is required.
> > 
> > Why not just set the PT_GNU_STACK flags correctly in the first place?
> 
> We do have PT_GNU_STACK flags set correctly, this feature is required to
> workaround CPU revisions which do not have RIXI support.

RIXI support can be discovered programatically from CP0_Config3.RXI
(cpu_has_rixi in asm/cpu-features.h), so I don't follow why CPUs without
RIXI would require a kernel parameter.

Cheers
James

> 
> Kind regards,
> Miodrag
> ________________________________________
> From: David Daney [ddaney@xxxxxxxxxxxxxxxxxx]
> Sent: Tuesday, November 21, 2017 9:53 PM
> To: Aleksandar Markovic; linux-mips@xxxxxxxxxxxxxx
> Cc: Miodrag Dinic; Aleksandar Markovic; Andrew Morton; DengCheng Zhu; Ding Tianhong; Douglas Leung; Frederic Weisbecker; Goran Ferenc; Ingo Molnar; James Cowgill; James Hogan; Jonathan Corbet; linux-doc@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; Marc Zyngier; Matt Redfearn; Mimi Zohar; Paul Burton; Paul E. McKenney; Petar Jovanovic; Raghu Gandham; Ralf Baechle; Thomas Gleixner; Tom Saeger
> Subject: Re: [PATCH v2] MIPS: Add nonxstack=on|off kernel parameter
> 
> On 11/21/2017 05:56 AM, Aleksandar Markovic wrote:
> > From: Miodrag Dinic <miodrag.dinic@xxxxxxxx>
> >
> > Add a new kernel parameter to override the default behavior related
> > to the decision whether to set up stack as non-executable in function
> > mips_elf_read_implies_exec().
> >
> > The new parameter is used to control non executable stack and heap,
> > regardless of PT_GNU_STACK entry. This does apply to both stack and
> > heap, despite the name.
> >
> > Allowed values:
> >
> > nonxstack=on  Force non-exec stack & heap
> > nonxstack=off Force executable stack & heap
> >
> > If this parameter is omitted, kernel behavior remains the same as it
> > was before this patch is applied.
> 
> Do other architectures have a similar hack?
> 
> If arm{,64} and x86 don't need this, what would make MIPS so special
> that we have to carry this around?
> 
> 
> >
> > This functionality is convenient during debugging and is especially
> > useful for Android development where non-exec stack is required.
> 
> Why not just set the PT_GNU_STACK flags correctly in the first place?
> 
> >
> > Signed-off-by: Miodrag Dinic <miodrag.dinic@xxxxxxxx>
> > Signed-off-by: Aleksandar Markovic <aleksandar.markovic@xxxxxxxx>
> > ---
> >   Documentation/admin-guide/kernel-parameters.txt | 11 +++++++
> >   arch/mips/kernel/elf.c                          | 39 +++++++++++++++++++++++++
> >   2 files changed, 50 insertions(+)
> >
> > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> > index b74e133..99464ee 100644
> > --- a/Documentation/admin-guide/kernel-parameters.txt
> > +++ b/Documentation/admin-guide/kernel-parameters.txt
> > @@ -2614,6 +2614,17 @@
> >                       noexec32=off: disable non-executable mappings
> >                               read implies executable mappings
> >
> > +     nonxstack       [MIPS]
> > +                     Force setting up stack and heap as non-executable or
> > +                     executable regardless of PT_GNU_STACK entry. Both
> > +                     stack and heap are affected, despite the name. Valid
> > +                     arguments: on, off.
> > +                     nonxstack=on:   Force non-executable stack and heap
> > +                     nonxstack=off:  Force executable stack and heap
> > +                     If ommited, stack and heap will or will not be set
> > +                     up as non-executable depending on PT_GNU_STACK
> > +                     entry and possibly other factors.
> > +
> >       nofpu           [MIPS,SH] Disable hardware FPU at boot time.
> >
> >       nofxsr          [BUGS=X86-32] Disables x86 floating point extended
> > diff --git a/arch/mips/kernel/elf.c b/arch/mips/kernel/elf.c
> > index 731325a..28ef7f3 100644
> > --- a/arch/mips/kernel/elf.c
> > +++ b/arch/mips/kernel/elf.c
> > @@ -326,8 +326,47 @@ void mips_set_personality_nan(struct arch_elf_state *state)
> >       }
> >   }
> >
> > +static int nonxstack = EXSTACK_DEFAULT;
> > +
> > +/*
> > + * kernel parameter: nonxstack=on|off
> > + *
> > + *   Force setting up stack and heap as non-executable or
> > + *   executable regardless of PT_GNU_STACK entry. Both
> > + *   stack and heap are affected, despite the name. Valid
> > + *   arguments: on, off.
> > + *
> > + *     nonxstack=on:   Force non-executable stack and heap
> > + *     nonxstack=off:  Force executable stack and heap
> > + *
> > + *   If ommited, stack and heap will or will not be set
> > + *   up as non-executable depending on PT_GNU_STACK
> > + *   entry and possibly other factors.
> > + */
> > +static int __init nonxstack_setup(char *str)
> > +{
> > +     if (!strcmp(str, "on"))
> > +             nonxstack = EXSTACK_DISABLE_X;
> > +     else if (!strcmp(str, "off"))
> > +             nonxstack = EXSTACK_ENABLE_X;
> > +     else
> > +             pr_err("Malformed nonxstack format! nonxstack=on|off\n");
> > +
> > +     return 1;
> > +}
> > +__setup("nonxstack=", nonxstack_setup);
> > +
> >   int mips_elf_read_implies_exec(void *elf_ex, int exstack)
> >   {
> > +     switch (nonxstack) {
> > +     case EXSTACK_DISABLE_X:
> > +             return 0;
> > +     case EXSTACK_ENABLE_X:
> > +             return 1;
> > +     default:
> > +             break;
> > +     }
> > +
> >       if (exstack != EXSTACK_DISABLE_X) {
> >               /* The binary doesn't request a non-executable stack */
> >               return 1;
> >
> 
>

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