arm64: uaccess: avoid blocking within critical sections
As Vincent reports in: https://lore.kernel.org/r/20211118163417.21617-1-vincent.whitchurch@axis.com The put_user() in schedule_tail() can get stuck in a livelock, similar to a problem recently fixed on riscv in commit: 285a76bb ("riscv: evaluate put_user() arg before enabling user access") In __raw_put_user() we have a critical section between uaccess_ttbr0_enable() and uaccess_ttbr0_disable() where we cannot safely call into the scheduler without having taken an exception, as schedule() and other scheduling functions will not save/restore the TTBR0 state. If either of the `x` or `ptr` arguments to __raw_put_user() contain a blocking call, we may call into the scheduler within the critical section. This can result in two problems: 1) The access within the critical section will occur without the required TTBR0 tables installed. This will fault, and where the required tables permit access, the access will be retried without the required tables, resulting in a livelock. 2) When TTBR0 SW PAN is in use, check_and_switch_context() does not modify TTBR0, leaving a stale value installed. The mappings of the blocked task will erroneously be accessible to regular accesses in the context of the new task. Additionally, if the tables are subsequently freed, local TLB maintenance required to reuse the ASID may be lost, potentially resulting in TLB corruption (e.g. in the presence of CnP). The same issue exists for __raw_get_user() in the critical section between uaccess_ttbr0_enable() and uaccess_ttbr0_disable(). A similar issue exists for __get_kernel_nofault() and __put_kernel_nofault() for the critical section between __uaccess_enable_tco_async() and __uaccess_disable_tco_async(), as the TCO state is not context-switched by direct calls into the scheduler. Here the TCO state may be lost from the context of the current task, resulting in unexpected asynchronous tag check faults. It may also be leaked to another task, suppressing expected tag check faults. To fix all of these cases, we must ensure that we do not directly call into the scheduler in their respective critical sections. This patch reworks __raw_put_user(), __raw_get_user(), __get_kernel_nofault(), and __put_kernel_nofault(), ensuring that parameters are evaluated outside of the critical sections. To make this requirement clear, comments are added describing the problem, and line spaces added to separate the critical sections from other portions of the macros. For __raw_get_user() and __raw_put_user() the `err` parameter is conditionally assigned to, and we must currently evaluate this in the critical section. This behaviour is relied upon by the signal code, which uses chains of put_user_error() and get_user_error(), checking the return value at the end. In all cases, the `err` parameter is a plain int rather than a more complex expression with a blocking call, so this is safe. In future we should try to clean up the `err` usage to remove the potential for this to be a problem. Aside from the changes to time of evaluation, there should be no functional change as a result of this patch. Reported-by:Vincent Whitchurch <vincent.whitchurch@axis.com> Link: https://lore.kernel.org/r/20211118163417.21617-1-vincent.whitchurch@axis.com Fixes: f253d827 ("arm64: uaccess: refactor __{get,put}_user") Signed-off-by:
Mark Rutland <mark.rutland@arm.com> Cc: Will Deacon <will@kernel.org> Cc: Catalin Marinas <catalin.marinas@arm.com> Link: https://lore.kernel.org/r/20211122125820.55286-1-mark.rutland@arm.comSigned-off-by:
Will Deacon <will@kernel.org>
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