[PATCH v2 0/7] Add machine check recovery when copying from user space

Tony Luck
2020-09-30T23:26:56Z

Machine check recovery from uncorrected memory errors currently focusses
primarily on errors that are detected while running in user mode. There
is a mechanism for recovering from errors in kernel code, but it is
currently only used for memcpy_mcsafe().

The existing recover actions for errors found in user mode (unmap the
page and send SIGBUS to the task) can also be applied when the error is
found while copying data from user space to the kernel.

Roadmap to this series:

Original part 0001 has already been applied to tip ras/core branch:
13c877f4b48b ("x86/mce: Stop mce_reign() from re-computing severity for every CPU")
so new part 0001 below used to be part 0002 in original series.

0001: First piece of infrastructure update. Severity calculations need
access to the saved registers. So pass pointer down the call
chain.

0002: Later we need to know what type of exception handler is present
for a given kernel instruction. Rather than proliferate more
functions like ex_has_fault_handler() for each type, replace
with a function that looks up the handler and returns an enum
describing the type.
Change since v1:
rename enum members s/HANDLER_NONE/EX_HANDLER_NONE/ etc.
drop __visible attribute

0003: Need slightly different handling for *copy_user*() faults from
get_user() faults. Create a new exception table tag and apply
to the copy functions.
Change since v1:
Add comments for kflags bits MCE_IN_KERNEL_RECOV and MCE_IN_KERNEL_COPYIN

0004: In fixup path of copy functions avoid dealing with the tail
when the copy took a machine check by returning that there
are no bytes left to be copied.
Change since v1:
Better comments in both commit and in code to explain the
reason for returning 0 == success, and why that is OK (process
will be sent a SIGBUS).

0005: For the REP MOVS copy case we need to check if %rsi is a user
or kernel address. There's already a static function to do this.
Make it global so x86/mce code can share the goodness.

0006: Changes to do_machine_check() to support the new recovery flow.
Some re-factoring to avoid code duplication (since the flows
for "error in user mode" and "error while copying from user
mode" are almost identical). Couple of new fields added to the
task structure.
Change since v1:
This patch used to have the code to decode the kernel
instruction and figure out what address was being accessed.
But doing that here was too late ... the severity code
needs to do that to distiguish copy to/from user in the
unrolled copy functions.

0007: Finally the keystone patch that pulls all the parts together.
An instruction decoder figures out whether an instruction
tagged as accessing user space is reading from or writing
to user space. The instructions in the switch were found
experimentally by looking at what instructions in the base
kernel are tagged in the exception table. I didn't add the
atomic operations (0x87 = XCHG etc.) that both read and write
user addresses. I think they should be safe, but I need a test
case where a futex has been poisoned to check. Probably this
switch should be expanded with all the instructions that the
compiler could possibly generate that read from user space.
Change since v1:
Moved the code to discover user address here in the
severity/context flow.

Tony Luck (3):
x86/mce: Provide method to find out the type of exception handle
x86/mce: Avoid tail copy when machine check terminated a copy from
user
x86/mce: Decode a kernel instruction to determine if it is copying
from user

Youquan Song (4):
x86/mce: Pass pointer to saved pt_regs to severity calculation
routines
x86/mce: Add _ASM_EXTABLE_CPY for copy user access
x86/mce: Change fault_in_kernel_space() from static to global
x86/mce: Recover from poison found while copying from user space

arch/x86/include/asm/asm.h | 6 ++
arch/x86/include/asm/extable.h | 9 ++-
arch/x86/include/asm/mce.h | 15 ++++
arch/x86/include/asm/traps.h | 2 +
arch/x86/kernel/cpu/mce/core.c | 58 +++++++++------
arch/x86/kernel/cpu/mce/internal.h | 3 +-
arch/x86/kernel/cpu/mce/severity.c | 68 ++++++++++++++++--
arch/x86/lib/copy_user_64.S | 111 ++++++++++++++++-------------
arch/x86/mm/extable.c | 24 +++++--
arch/x86/mm/fault.c | 2 +-
include/linux/sched.h | 2 +
11 files changed, 216 insertions(+), 84 deletions(-)

base-commit: dc0592b73715c8e84ad8ebbc50c6057d5e203aac
--
2.21.1