RE: [PATCH v2] MIPS: Add nonxstack=on|off kernel parameter
From: Miodrag Dinic <Miodrag.Dinic@mips.com>
Date: 2017-11-30 09:35:41
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Hi David, Sorry for a late response, please find answers in-lined:
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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
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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. Kind regards, Miodrag ________________________________________ From: David Daney [ddaney@caviumnetworks.com] Sent: Tuesday, November 21, 2017 9:53 PM To: Aleksandar Markovic; linux-mips@linux-mips.org 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@vger.kernel.org; linux-kernel@vger.kernel.org; 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@mips.com> 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?
quoted hunk ↗ jump to hunk
Signed-off-by: Miodrag Dinic <miodrag.dinic@mips.com> Signed-off-by: Aleksandar Markovic <aleksandar.markovic@mips.com> --- 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 extendeddiff --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;