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powerpc/dexcr: Reset DEXCR value across exec

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Inheriting the DEXCR across exec can have security and usability
concerns. If a program is compiled with hash instructions it generally
expects to run with NPHIE enabled. But if the parent process disables
NPHIE then if it's not careful it will be disabled for any children too
and the protection offered by hash checks is basically worthless.

This patch introduces a per-process reset value that new execs in a
particular process tree are initialized with. This enables fine grained
control over what DEXCR value child processes run with by default.
For example, containers running legacy binaries that expect hash
instructions to act as NOPs could configure the reset value of the
container root to control the default reset value for all members of
the container.

Signed-off-by: Benjamin Gray <bgray@linux.ibm.com>
[mpe: Add missing SPDX tag on dexcr.c]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240417112325.728010-4-bgray@linux.ibm.com
This commit is contained in:
Benjamin Gray 2024-04-17 21:23:19 +10:00 committed by Michael Ellerman
parent 75171f06c4
commit bbd99922d0
4 changed files with 32 additions and 1 deletions
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2
arch/powerpc/include/asm/processor.h
View File

@ -261,7 +261,7 @@ struct thread_struct {
unsigned long sier3;
unsigned long hashkeyr;
unsigned long dexcr;
unsigned long dexcr_onexec; /* Reset value to load on exec */
#endif
};

1
arch/powerpc/kernel/Makefile
View File

@ -87,6 +87,7 @@ obj-$(CONFIG_HAVE_HW_BREAKPOINT) += hw_breakpoint.o
obj-$(CONFIG_PPC_DAWR) += dawr.o
obj-$(CONFIG_PPC_BOOK3S_64) += cpu_setup_ppc970.o cpu_setup_pa6t.o
obj-$(CONFIG_PPC_BOOK3S_64) += cpu_setup_power.o
obj-$(CONFIG_PPC_BOOK3S_64) += dexcr.o
obj-$(CONFIG_PPC_BOOK3S_64) += mce.o mce_power.o
obj-$(CONFIG_PPC_BOOK3E_64) += exceptions-64e.o idle_64e.o
obj-$(CONFIG_PPC_BARRIER_NOSPEC) += security.o

23
arch/powerpc/kernel/dexcr.c Normal file
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@ -0,0 +1,23 @@
// SPDX-License-Identifier: GPL-2.0-or-later
#include <linux/capability.h>
#include <linux/cpu.h>
#include <linux/init.h>
#include <linux/prctl.h>
#include <linux/sched.h>
#include <asm/cpu_has_feature.h>
#include <asm/cputable.h>
#include <asm/processor.h>
#include <asm/reg.h>
static int __init init_task_dexcr(void)
{
if (!early_cpu_has_feature(CPU_FTR_ARCH_31))
return 0;
current->thread.dexcr_onexec = mfspr(SPRN_DEXCR);
return 0;
}
early_initcall(init_task_dexcr)

7
arch/powerpc/kernel/process.c
View File

@ -1641,6 +1641,13 @@ void arch_setup_new_exec(void)
current->thread.regs->amr = default_amr;
current->thread.regs->iamr = default_iamr;
#endif
#ifdef CONFIG_PPC_BOOK3S_64
if (cpu_has_feature(CPU_FTR_ARCH_31)) {
current->thread.dexcr = current->thread.dexcr_onexec;
mtspr(SPRN_DEXCR, current->thread.dexcr);
}
#endif /* CONFIG_PPC_BOOK3S_64 */
}
#ifdef CONFIG_PPC64