[PATCH v8 14/19] arm64: exec: Adjust affinity for compat tasks with mismatched 32-bit EL0

Will Deacon
2021-06-02T16:57:14Z

When exec'ing a 32-bit task on a system with mismatched support for
32-bit EL0, try to ensure that it starts life on a CPU that can actually
run it.

Similarly, when exec'ing a 64-bit task on such a system, try to restore
the old affinity mask if it was previously restricted.

Reviewed-by: Quentin Perret <qperret@google.com>
Signed-off-by: Will Deacon <will@kernel.org>
---
arch/arm64/include/asm/elf.h | 6 ++----
arch/arm64/kernel/process.c | 39 +++++++++++++++++++++++++++++++++++-
2 files changed, 40 insertions(+), 5 deletions(-)

diff --git a/arch/arm64/include/asm/elf.h b/arch/arm64/include/asm/elf.h
index 8d1c8dcb87fd..97932fbf973d 100644
--- a/arch/arm64/include/asm/elf.h
+++ b/arch/arm64/include/asm/elf.h
@@ -213,10 +213,8 @@ typedef compat_elf_greg_t compat_elf_gregset_t[COMPAT_ELF_NGREG];

/* AArch32 EABI. */
#define EF_ARM_EABI_MASK 0xff000000
-#define compat_elf_check_arch(x) (system_supports_32bit_el0() && \
- ((x)->e_machine == EM_ARM) && \
- ((x)->e_flags & EF_ARM_EABI_MASK))
-
+int compat_elf_check_arch(const struct elf32_hdr *);
+#define compat_elf_check_arch compat_elf_check_arch
#define compat_start_thread compat_start_thread
/*
* Unlike the native SET_PERSONALITY macro, the compat version maintains
diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
index f4a91bf1ce0c..3aea06fdd1f9 100644
--- a/arch/arm64/kernel/process.c
+++ b/arch/arm64/kernel/process.c
@@ -22,6 +22,7 @@
#include <linux/mman.h>
#include <linux/mm.h>
#include <linux/nospec.h>
+#include <linux/sched.h>
#include <linux/stddef.h>
#include <linux/sysctl.h>
#include <linux/unistd.h>
@@ -638,6 +639,28 @@ unsigned long arch_align_stack(unsigned long sp)
return sp & ~0xf;
}

+#ifdef CONFIG_COMPAT
+int compat_elf_check_arch(const struct elf32_hdr *hdr)
+{
+ if (!system_supports_32bit_el0())
+ return false;
+
+ if ((hdr)->e_machine != EM_ARM)
+ return false;
+
+ if (!((hdr)->e_flags & EF_ARM_EABI_MASK))
+ return false;
+
+ /*
+ * Prevent execve() of a 32-bit program from a deadline task
+ * if the restricted affinity mask would be inadmissible on an
+ * asymmetric system.
+ */
+ return !static_branch_unlikely(&arm64_mismatched_32bit_el0) ||
+ task_cpus_dl_admissible(current, system_32bit_el0_cpumask());
+}
+#endif
+
/*
* Called from setup_new_exec() after (COMPAT_)SET_PERSONALITY.
*/
@@ -647,8 +670,22 @@ void arch_setup_new_exec(void)

if (is_compat_task()) {
mmflags = MMCF_AARCH32;
- if (static_branch_unlikely(&arm64_mismatched_32bit_el0))
+
+ /*
+ * Restrict the CPU affinity mask for a 32-bit task so that
+ * it contains only 32-bit-capable CPUs.
+ *
+ * From the perspective of the task, this looks similar to
+ * what would happen if the 64-bit-only CPUs were hot-unplugged
+ * at the point of execve(), although we try a bit harder to
+ * honour the cpuset hierarchy.
+ */
+ if (static_branch_unlikely(&arm64_mismatched_32bit_el0)) {
+ force_compatible_cpus_allowed_ptr(current);
set_tsk_thread_flag(current, TIF_NOTIFY_RESUME);
+ }
+ } else if (static_branch_unlikely(&arm64_mismatched_32bit_el0)) {
+ relax_compatible_cpus_allowed_ptr(current);
}

current->mm->context.flags = mmflags;
--
2.32.0.rc0.204.g9fa02ecfa5-goog