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SubjectRe: [PATCH] cfi: Fix __cfi_slowpath_diag RCU usage with cpuidle
On Tue, May 31, 2022 at 10:59:10AM -0700, Sami Tolvanen wrote:
> RCU_NONIDLE usage during __cfi_slowpath_diag can result in an invalid
> RCU state in the cpuidle code path:
>
> WARNING: CPU: 1 PID: 0 at kernel/rcu/tree.c:613 rcu_eqs_enter+0xe4/0x138
> ...
> Call trace:
> rcu_eqs_enter+0xe4/0x138
> rcu_idle_enter+0xa8/0x100
> cpuidle_enter_state+0x154/0x3a8
> cpuidle_enter+0x3c/0x58
> do_idle.llvm.6590768638138871020+0x1f4/0x2ec
> cpu_startup_entry+0x28/0x2c
> secondary_start_kernel+0x1b8/0x220
> __secondary_switched+0x94/0x98
>
> Instead, call rcu_irq_enter/exit to wake up RCU only when needed and
> disable interrupts for the entire CFI shadow/module check when we do.
>
> Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
> ---
> kernel/cfi.c | 22 ++++++++++++++++------
> 1 file changed, 16 insertions(+), 6 deletions(-)
>
> diff --git a/kernel/cfi.c b/kernel/cfi.c
> index 9594cfd1cf2c..08102d19ec15 100644
> --- a/kernel/cfi.c
> +++ b/kernel/cfi.c
> @@ -281,6 +281,8 @@ static inline cfi_check_fn find_module_check_fn(unsigned long ptr)
> static inline cfi_check_fn find_check_fn(unsigned long ptr)
> {
> cfi_check_fn fn = NULL;
> + unsigned long flags;
> + bool rcu_idle;
>
> if (is_kernel_text(ptr))
> return __cfi_check;
> @@ -290,13 +292,21 @@ static inline cfi_check_fn find_check_fn(unsigned long ptr)
> * the shadow and __module_address use RCU, so we need to wake it
> * up if necessary.
> */
> - RCU_NONIDLE({
> - if (IS_ENABLED(CONFIG_CFI_CLANG_SHADOW))
> - fn = find_shadow_check_fn(ptr);
> + rcu_idle = !rcu_is_watching();
> + if (rcu_idle) {
> + local_irq_save(flags);
> + rcu_irq_enter();
> + }
> +
> + if (IS_ENABLED(CONFIG_CFI_CLANG_SHADOW))
> + fn = find_shadow_check_fn(ptr);
> + if (!fn)
> + fn = find_module_check_fn(ptr);
>
> - if (!fn)
> - fn = find_module_check_fn(ptr);
> - });
> + if (rcu_idle) {
> + rcu_irq_exit();
> + local_irq_restore(flags);
> + }
>
> return fn;
> }
> --
> 2.36.1.255.ge46751e96f-goog
>

Thanks for the fix! It looks like these tags could be added, yes?

Fixes: cf68fffb66d6 ("add support for Clang CFI")
Cc: stable@vger.kernel.org

Also, to improve the commit log, under what situation was this problem
encountered?

--
Kees Cook

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