# git rev-parse -q --verify 8150a153c013aa2dd1ffae43370b89ac1347a7fb^{commit}
8150a153c013aa2dd1ffae43370b89ac1347a7fb
already have revision, skipping fetch
# git checkout -q -f -B kisskb 8150a153c013aa2dd1ffae43370b89ac1347a7fb
# git clean -qxdf
# < git log -1
# commit 8150a153c013aa2dd1ffae43370b89ac1347a7fb
# Author: Michael Ellerman <mpe@ellerman.id.au>
# Date:   Wed May 8 13:06:42 2019 +1000
# 
#     powerpc/64s: Use early_mmu_has_feature() in set_kuap()
#     
#     When implementing the KUAP support on Radix we fixed one case where
#     mmu_has_feature() was being called too early in boot via
#     __put_user_size().
#     
#     However since then some new code in linux-next has created a new path
#     via which we can end up calling mmu_has_feature() too early.
#     
#     On P9 this leads to crashes early in boot if we have both PPC_KUAP and
#     CONFIG_JUMP_LABEL_FEATURE_CHECK_DEBUG enabled. Our early boot code
#     calls printk() which calls probe_kernel_read(), that does a
#     __copy_from_user_inatomic() which calls into set_kuap() and that uses
#     mmu_has_feature().
#     
#     At that point in boot we haven't patched MMU features yet so the debug
#     code in mmu_has_feature() complains, and calls printk(). At that point
#     we recurse, eg:
#     
#       ...
#       dump_stack+0xdc
#       probe_kernel_read+0x1a4
#       check_pointer+0x58
#       ...
#       printk+0x40
#       dump_stack_print_info+0xbc
#       dump_stack+0x8
#       probe_kernel_read+0x1a4
#       probe_kernel_read+0x19c
#       check_pointer+0x58
#       ...
#       printk+0x40
#       cpufeatures_process_feature+0xc8
#       scan_cpufeatures_subnodes+0x380
#       of_scan_flat_dt_subnodes+0xb4
#       dt_cpu_ftrs_scan_callback+0x158
#       of_scan_flat_dt+0xf0
#       dt_cpu_ftrs_scan+0x3c
#       early_init_devtree+0x360
#       early_setup+0x9c
#     
#     And so on for infinity, symptom is a dead system.
#     
#     Even more fun is what happens when using the hash MMU (ie. p8 or p9
#     with Radix disabled), and when we don't have
#     CONFIG_JUMP_LABEL_FEATURE_CHECK_DEBUG enabled. With the debug disabled
#     we don't check if static keys have been initialised, we just rely on
#     the jump label. But the jump label defaults to true so we just whack
#     the AMR even though Radix is not enabled.
#     
#     Clearing the AMR is fine, but after we've done the user copy we write
#     (0b11 << 62) into AMR. When using hash that makes all pages with key
#     zero no longer readable or writable. All kernel pages implicitly have
#     key zero, and so all of a sudden the kernel can't read or write any of
#     its memory. Again dead system.
#     
#     In the medium term we have several options for fixing this.
#     probe_kernel_read() doesn't need to touch AMR at all, it's not doing a
#     user access after all, but it uses __copy_from_user_inatomic() just
#     because it's easy, we could fix that.
#     
#     It would also be safe to default to not writing to the AMR during
#     early boot, until we've detected features. But it's not clear that
#     flipping all the MMU features to static_key_false won't introduce
#     other bugs.
#     
#     But for now just switch to early_mmu_has_feature() in set_kuap(), that
#     avoids all the problems with jump labels. It adds the overhead of a
#     global lookup and test, but that's probably trivial compared to the
#     writes to the AMR anyway.
#     
#     Fixes: 890274c2dc4c ("powerpc/64s: Implement KUAP for Radix MMU")
#     Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
#     Reviewed-by: Russell Currey <ruscur@russell.cc>
# < /opt/cross/kisskb/korg/gcc-5.5.0-nolibc/powerpc64-linux/bin/powerpc64-linux-gcc --version
# < /opt/cross/kisskb/korg/gcc-5.5.0-nolibc/powerpc64-linux/bin/powerpc64-linux-ld --version
# < git log --format=%s --max-count=1 8150a153c013aa2dd1ffae43370b89ac1347a7fb
# < make -s -j 120 ARCH=powerpc O=/kisskb/build/powerpc-next_83xx_mpc837x_rdb_defconfig_powerpc-gcc5 CROSS_COMPILE=/opt/cross/kisskb/korg/gcc-5.5.0-nolibc/powerpc64-linux/bin/powerpc64-linux-  83xx/mpc837x_rdb_defconfig
# make -s -j 120 ARCH=powerpc O=/kisskb/build/powerpc-next_83xx_mpc837x_rdb_defconfig_powerpc-gcc5 CROSS_COMPILE=/opt/cross/kisskb/korg/gcc-5.5.0-nolibc/powerpc64-linux/bin/powerpc64-linux-  
<stdin>:1478:2: warning: #warning syscall pidfd_send_signal not implemented [-Wcpp]
<stdin>:1481:2: warning: #warning syscall io_uring_setup not implemented [-Wcpp]
<stdin>:1484:2: warning: #warning syscall io_uring_enter not implemented [-Wcpp]
<stdin>:1487:2: warning: #warning syscall io_uring_register not implemented [-Wcpp]
/kisskb/src/kernel/futex.c: In function 'do_futex':
/kisskb/src/kernel/futex.c:1658:17: warning: 'oldval' may be used uninitialized in this function [-Wmaybe-uninitialized]
   return oldval == cmparg;
                 ^
/kisskb/src/kernel/futex.c:1633:6: note: 'oldval' was declared here
  int oldval, ret;
      ^
INFO: Uncompressed kernel (size 0x60aadc) overlaps the address of the wrapper(0x400000)
INFO: Fixing the link_address of wrapper to (0x700000)
Image Name:   Linux-5.1.0-rc3-g8150a153c013
Created:      Fri May 10 03:36:13 2019
Image Type:   PowerPC Linux Kernel Image (gzip compressed)
Data Size:    3151374 Bytes = 3077.51 KiB = 3.01 MiB
Load Address: 00000000
Entry Point:  00000000
Completed OK
# rm -rf /kisskb/build/powerpc-next_83xx_mpc837x_rdb_defconfig_powerpc-gcc5
# Build took: 0:00:18.254478