s390/mm: fix asce_bits handling with dynamic pagetable levels

[bugproxy]

== Comment: #0 - Hendrik Brueckner <email address hidden> - 2016-05-23 09:17:08 ==
Please backport the following linux stable commit ID:

linux-stable: https://git.kernel.org/cgit/linux/kernel/git/stable/linux-stable.git/commit/?h=linux-4.4.y&id=ce1bc448bac01edfccdc26d8318cfd39aa09e6e0

s390/mm: fix asce_bits handling with dynamic pagetable levels
commit 723cacbd9dc79582e562c123a0bacf8bfc69e72a upstream.

There is a race with multi-threaded applications between context switch and
pagetable upgrade. In switch_mm() a new user_asce is built from mm->pgd and
mm->context.asce_bits, w/o holding any locks. A concurrent mmap with a
pagetable upgrade on another thread in crst_table_upgrade() could already
have set new asce_bits, but not yet the new mm->pgd. This would result in a
corrupt user_asce in switch_mm(), and eventually in a kernel panic from a
translation exception.

Fix this by storing the complete asce instead of just the asce_bits, which
can then be read atomically from switch_mm(), so that it either sees the
old value or the new value, but no mixture. Both cases are OK. Having the
old value would result in a page fault on access to the higher level memory,
but the fault handler would see the new mm->pgd, if it was a valid access
after the mmap on the other thread has completed. So as worst-case scenario
we would have a page fault loop for the racing thread until the next time
slice.

Also remove dead code and simplify the upgrade/downgrade path, there are no
upgrades from 2 levels, and only downgrades from 3 levels for compat tasks.
There are also no concurrent upgrades, because the mmap_sem is held with
down_write() in do_mmap, so the flush and table checks during upgrade can
be removed.

Reported-by: Michael Munday <email address hidden>
Reviewed-by: Martin Schwidefsky <email address hidden>
Signed-off-by: Gerald Schaefer <email address hidden>
Signed-off-by: Martin Schwidefsky <email address hidden>
Signed-off-by: Greg Kroah-Hartman <email address hidden>

[Tim Gardner (timg-tpi)]

Released in v4.4.11. This patch will get merged in Ubuntu-4.4.0-24.42 via the normal stable updates process.

[Kamal Mostafa (kamalmostafa)]

This bug is awaiting verification that the kernel in -proposed solves the problem. Please test the kernel and update this bug with the results. If the problem is solved, change the tag 'verification-needed-xenial' to 'verification-done-xenial'.

If verification is not done by 5 working days from today, this fix will be dropped from the source code, and this bug will be closed.

See https://wiki.ubuntu.com/Testing/EnableProposed for documentation how to enable and use -proposed. Thank you!

[bugproxy (bugproxy)]

------- Comment From <email address hidden> 2016-06-14 12:30 EDT-------
I verified that the kernel in -proposed (4.4.0-25.44) fixes the problem.

[Launchpad Janitor (janitor)]

This bug was fixed in the package linux - 4.4.0-28.47

---------------
linux (4.4.0-28.47) xenial; urgency=low

  [ Luis Henriques ]

  * Release Tracking Bug
    - LP: #1595874

  * Linux netfilter local privilege escalation issues (LP: #1595350)
    - netfilter: x_tables: don't move to non-existent next rule
    - netfilter: x_tables: validate targets of jumps
    - netfilter: x_tables: add and use xt_check_entry_offsets
    - netfilter: x_tables: kill check_entry helper
    - netfilter: x_tables: assert minimum target size
    - netfilter: x_tables: add compat version of xt_check_entry_offsets
    - netfilter: x_tables: check standard target size too
    - netfilter: x_tables: check for bogus target offset
    - netfilter: x_tables: validate all offsets and sizes in a rule
    - netfilter: x_tables: don't reject valid target size on some architectures
    - netfilter: arp_tables: simplify translate_compat_table args
    - netfilter: ip_tables: simplify translate_compat_table args
    - netfilter: ip6_tables: simplify translate_compat_table args
    - netfilter: x_tables: xt_compat_match_from_user doesn't need a retval
    - netfilter: x_tables: do compat validation via translate_table
    - netfilter: x_tables: introduce and use xt_copy_counters_from_user

  * Linux netfilter IPT_SO_SET_REPLACE memory corruption (LP: #1555338)
    - netfilter: x_tables: validate e->target_offset early
    - netfilter: x_tables: make sure e->next_offset covers remaining blob size
    - netfilter: x_tables: fix unconditional helper

linux (4.4.0-27.46) xenial; urgency=low

  [ Kamal Mostafa ]

  * Release Tracking Bug
    - LP: #1594906

  * Support Edge Gateway's Bluetooth LED (LP: #1512999)
    - Revert "UBUNTU: SAUCE: Bluetooth: Support for LED on Marvell modules"

linux (4.4.0-26.45) xenial; urgency=low

  [ Kamal Mostafa ]

  * Release Tracking Bug
    - LP: #1594442

  * linux: Implement secure boot state variables (LP: #1593075)
    - SAUCE: UEFI: Add secure boot and MOK SB State disabled sysctl

  * failures building userspace packages that include ethtool.h (LP: #1592930)
    - ethtool.h: define INT_MAX for userland

linux (4.4.0-25.44) xenial; urgency=low

  [ Kamal Mostafa ]

  * Release Tracking Bug
    - LP: #1591289

  * Xenial update to v4.4.13 stable release (LP: #1590455)
    - MIPS64: R6: R2 emulation bugfix
    - MIPS: math-emu: Fix jalr emulation when rd == $0
    - MIPS: MSA: Fix a link error on `_init_msa_upper' with older GCC
    - MIPS: Don't unwind to user mode with EVA
    - MIPS: Avoid using unwind_stack() with usermode
    - MIPS: Fix siginfo.h to use strict posix types
    - MIPS: Fix uapi include in exported asm/siginfo.h
    - MIPS: Fix watchpoint restoration
    - MIPS: Flush highmem pages in __flush_dcache_page
    - MIPS: Handle highmem pages in __update_cache
    - MIPS: Sync icache & dcache in set_pte_at
    - MIPS: ath79: make bootconsole wait for both THRE and TEMT
    - MIPS: Reserve nosave data for hibernation
    - MIPS: Loongson-3: Reserve 32MB for RS780E integrated GPU
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    - MIPS: Fix MSA ld_*/st_* asm macros to use PTR_ADDU
    - MIPS: Prevent "restoration" of MSA c...