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arm64: kpti: Fix the interaction between ASID switching and software PAN

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With ARM64_SW_TTBR0_PAN enabled, the exception entry code checks the
active ASID to decide whether user access was enabled (non-zero ASID)
when the exception was taken. On return from exception, if user access
was previously disabled, it re-instates TTBR0_EL1 from the per-thread
saved value (updated in switch_mm() or efi_set_pgd()).

Commit 7655abb953 ("arm64: mm: Move ASID from TTBR0 to TTBR1") makes a
TTBR0_EL1 + ASID switching non-atomic. Subsequently, commit 27a921e757
("arm64: mm: Fix and re-enable ARM64_SW_TTBR0_PAN") changes the
__uaccess_ttbr0_disable() function and asm macro to first write the
reserved TTBR0_EL1 followed by the ASID=0 update in TTBR1_EL1. If an
exception occurs between these two, the exception return code will
re-instate a valid TTBR0_EL1. Similar scenario can happen in
cpu_switch_mm() between setting the reserved TTBR0_EL1 and the ASID
update in cpu_do_switch_mm().

This patch reverts the entry.S check for ASID == 0 to TTBR0_EL1 and
disables the interrupts around the TTBR0_EL1 and ASID switching code in
__uaccess_ttbr0_disable(). It also ensures that, when returning from the
EFI runtime services, efi_set_pgd() doesn't leave a non-zero ASID in
TTBR1_EL1 by using uaccess_ttbr0_{enable,disable}.

The accesses to current_thread_info()->ttbr0 are updated to use
READ_ONCE/WRITE_ONCE.

As a safety measure, __uaccess_ttbr0_enable() always masks out any
existing non-zero ASID TTBR1_EL1 before writing in the new ASID.

Fixes: 27a921e757 ("arm64: mm: Fix and re-enable ARM64_SW_TTBR0_PAN")
Acked-by: Will Deacon <will.deacon@arm.com>
Reported-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Tested-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: James Morse <james.morse@arm.com>
Tested-by: James Morse <james.morse@arm.com>
Co-developed-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
This commit is contained in:
Catalin Marinas
2018-01-10 13:18:30 +00:00
parent 558daf693e
commit 6b88a32c7a
12 changed files with 32 additions and 21 deletions
Download Patch File Download Diff File Expand all files Collapse all files

12
arch/arm64/include/asm/efi.h
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@@ -121,19 +121,21 @@ static inline void efi_set_pgd(struct mm_struct *mm)
if (mm != current->active_mm) {
/*
* Update the current thread's saved ttbr0 since it is
* restored as part of a return from exception. Set
* the hardware TTBR0_EL1 using cpu_switch_mm()
* directly to enable potential errata workarounds.
* restored as part of a return from exception. Enable
* access to the valid TTBR0_EL1 and invoke the errata
* workaround directly since there is no return from
* exception when invoking the EFI run-time services.
*/
update_saved_ttbr0(current, mm);
cpu_switch_mm(mm->pgd, mm);
uaccess_ttbr0_enable();
post_ttbr_update_workaround();
} else {
/*
* Defer the switch to the current thread's TTBR0_EL1
* until uaccess_enable(). Restore the current
* thread's saved ttbr0 corresponding to its active_mm
*/
cpu_set_reserved_ttbr0();
uaccess_ttbr0_disable();
update_saved_ttbr0(current, current->active_mm);
}
}