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[PATCH 3.18 07/24] arm64: mm: abort uaccess retries upon fatal signal

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3.18-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Mark Rutland <mark.rutland@arm.com>

commit 289d07a2dc6c6b6f3e4b8a62669320d99dbe6c3d upstream.

When there's a fatal signal pending, arm64's do_page_fault()
implementation returns 0. The intent is that we'll return to the
faulting userspace instruction, delivering the signal on the way.

However, if we take a fatal signal during fixing up a uaccess, this
results in a return to the faulting kernel instruction, which will be
instantly retried, resulting in the same fault being taken forever. As
the task never reaches userspace, the signal is not delivered, and the
task is left unkillable. While the task is stuck in this state, it can
inhibit the forward progress of the system.

To avoid this, we must ensure that when a fatal signal is pending, we
apply any necessary fixup for a faulting kernel instruction. Thus we
will return to an error path, and it is up to that code to make forward
progress towards delivering the fatal signal.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Laura Abbott <labbott@redhat.com>
Reviewed-by: Steve Capper <steve.capper@arm.com>
Tested-by: Steve Capper <steve.capper@arm.com>
Reviewed-by: James Morse <james.morse@arm.com>
Tested-by: James Morse <james.morse@arm.com>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 arch/arm64/mm/fault.c |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

--- a/arch/arm64/mm/fault.c
+++ b/arch/arm64/mm/fault.c
@@ -253,8 +253,11 @@ retry:
 	 * signal first. We do not need to release the mmap_sem because it
 	 * would already be released in __lock_page_or_retry in mm/filemap.c.
 	 */
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current)) {
+		if (!user_mode(regs))
+			goto no_context;
 		return 0;
+	}
 
 	/*
 	 * Major/minor page fault accounting is only done on the initial

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[PATCH 3.18 00/24] 3.18.69-stable review Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-08-31 17:50 +0200
  [PATCH 3.18 15/24] mm: cma: fix incorrect type conversion for size during dma allocation Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-08-31 17:50 +0200
  [PATCH 3.18 11/24] mm: cma: split cma-reserved in dmesg log Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-08-31 17:50 +0200
  [PATCH 3.18 05/24] gcov: support GCC 7.1 Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-08-31 17:50 +0200
  [PATCH 3.18 23/24] scsi: sg: reset res_in_use after unlinking reserved array Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-08-31 17:50 +0200
  [PATCH 3.18 17/24] arm64: flush FP/SIMD state correctly after execve() Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-08-31 17:50 +0200
  [PATCH 3.18 19/24] x86-64: Handle PC-relative relocations on per-CPU data Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-08-31 17:50 +0200
  [PATCH 3.18 02/24] ALSA: au88x0: Fix zero clear of stream->resources Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-08-31 18:00 +0200
  [PATCH 3.18 18/24] arm64: fpsimd: Prevent registers leaking across exec Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-08-31 18:00 +0200
  [PATCH 3.18 10/24] mm/cma: make kmemleak ignore CMA regions Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-08-31 18:00 +0200
  [PATCH 3.18 12/24] mm: cma: fix totalcma_pages to include DT defined CMA regions Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-08-31 18:00 +0200
  [PATCH 3.18 22/24] scsi: sg: protect accesses to reserved page array Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-08-31 18:00 +0200
  [PATCH 3.18 03/24] gcov: add support for GCC 5.1 Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-08-31 18:00 +0200
  [PATCH 3.18 14/24] mm: cma: constify and use correct signness in mm/cma.c Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-08-31 18:00 +0200
  [PATCH 3.18 09/24] mm: cma: align to physical address, not CMA region position Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-08-31 18:00 +0200
  [PATCH 3.18 04/24] gcov: add support for gcc version >= 6 Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-08-31 18:00 +0200
  [PATCH 3.18 07/24] arm64: mm: abort uaccess retries upon fatal signal Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-08-31 18:00 +0200
  Re: [PATCH 3.18 00/24] 3.18.69-stable review Shuah Khan <shuahkh@osg.samsung.com> - 2017-08-31 21:10 +0200
  Re: [PATCH 3.18 00/24] 3.18.69-stable review Guenter Roeck <linux@roeck-us.net> - 2017-08-31 21:40 +0200
  Re: [PATCH 3.18 00/24] 3.18.69-stable review Guenter Roeck <linux@roeck-us.net> - 2017-09-01 04:40 +0200
    Re: [PATCH 3.18 00/24] 3.18.69-stable review Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-09-01 07:10 +0200

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