6.6-stable review patch. If anyone has any objections, please let me know.
------------------
From: Marco Elver elver@google.com
commit f6564fce256a3944aa1bc76cb3c40e792d97c1eb upstream.
Alexander Potapenko writes in [1]: "For every memory access in the code instrumented by KMSAN we call kmsan_get_metadata() to obtain the metadata for the memory being accessed. For virtual memory the metadata pointers are stored in the corresponding `struct page`, therefore we need to call virt_to_page() to get them.
According to the comment in arch/x86/include/asm/page.h, virt_to_page(kaddr) returns a valid pointer iff virt_addr_valid(kaddr) is true, so KMSAN needs to call virt_addr_valid() as well.
To avoid recursion, kmsan_get_metadata() must not call instrumented code, therefore ./arch/x86/include/asm/kmsan.h forks parts of arch/x86/mm/physaddr.c to check whether a virtual address is valid or not.
But the introduction of rcu_read_lock() to pfn_valid() added instrumented RCU API calls to virt_to_page_or_null(), which is called by kmsan_get_metadata(), so there is an infinite recursion now. I do not think it is correct to stop that recursion by doing kmsan_enter_runtime()/kmsan_exit_runtime() in kmsan_get_metadata(): that would prevent instrumented functions called from within the runtime from tracking the shadow values, which might introduce false positives."
Fix the issue by switching pfn_valid() to the _sched() variant of rcu_read_lock/unlock(), which does not require calling into RCU. Given the critical section in pfn_valid() is very small, this is a reasonable trade-off (with preemptible RCU).
KMSAN further needs to be careful to suppress calls into the scheduler, which would be another source of recursion. This can be done by wrapping the call to pfn_valid() into preempt_disable/enable_no_resched(). The downside is that this sacrifices breaking scheduling guarantees; however, a kernel compiled with KMSAN has already given up any performance guarantees due to being heavily instrumented.
Note, KMSAN code already disables tracing via Makefile, and since mmzone.h is included, it is not necessary to use the notrace variant, which is generally preferred in all other cases.
Link: https://lkml.kernel.org/r/20240115184430.2710652-1-glider@google.com [1] Link: https://lkml.kernel.org/r/20240118110022.2538350-1-elver@google.com Fixes: 5ec8e8ea8b77 ("mm/sparsemem: fix race in accessing memory_section->usage") Signed-off-by: Marco Elver elver@google.com Reported-by: Alexander Potapenko glider@google.com Reported-by: syzbot+93a9e8a3dea8d6085e12@syzkaller.appspotmail.com Reviewed-by: Alexander Potapenko glider@google.com Tested-by: Alexander Potapenko glider@google.com Cc: Charan Teja Kalla quic_charante@quicinc.com Cc: Borislav Petkov (AMD) bp@alien8.de Cc: Dave Hansen dave.hansen@linux.intel.com Cc: Dmitry Vyukov dvyukov@google.com Cc: "H. Peter Anvin" hpa@zytor.com Cc: Ingo Molnar mingo@redhat.com Cc: Thomas Gleixner tglx@linutronix.de Signed-off-by: Andrew Morton akpm@linux-foundation.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- arch/x86/include/asm/kmsan.h | 17 ++++++++++++++++- include/linux/mmzone.h | 6 +++--- 2 files changed, 19 insertions(+), 4 deletions(-)
--- a/arch/x86/include/asm/kmsan.h +++ b/arch/x86/include/asm/kmsan.h @@ -64,6 +64,7 @@ static inline bool kmsan_virt_addr_valid { unsigned long x = (unsigned long)addr; unsigned long y = x - __START_KERNEL_map; + bool ret;
/* use the carry flag to determine if x was < __START_KERNEL_map */ if (unlikely(x > y)) { @@ -79,7 +80,21 @@ static inline bool kmsan_virt_addr_valid return false; }
- return pfn_valid(x >> PAGE_SHIFT); + /* + * pfn_valid() relies on RCU, and may call into the scheduler on exiting + * the critical section. However, this would result in recursion with + * KMSAN. Therefore, disable preemption here, and re-enable preemption + * below while suppressing reschedules to avoid recursion. + * + * Note, this sacrifices occasionally breaking scheduling guarantees. + * Although, a kernel compiled with KMSAN has already given up on any + * performance guarantees due to being heavily instrumented. + */ + preempt_disable(); + ret = pfn_valid(x >> PAGE_SHIFT); + preempt_enable_no_resched(); + + return ret; }
#endif /* !MODULE */ --- a/include/linux/mmzone.h +++ b/include/linux/mmzone.h @@ -2006,9 +2006,9 @@ static inline int pfn_valid(unsigned lon if (pfn_to_section_nr(pfn) >= NR_MEM_SECTIONS) return 0; ms = __pfn_to_section(pfn); - rcu_read_lock(); + rcu_read_lock_sched(); if (!valid_section(ms)) { - rcu_read_unlock(); + rcu_read_unlock_sched(); return 0; } /* @@ -2016,7 +2016,7 @@ static inline int pfn_valid(unsigned lon * the entire section-sized span. */ ret = early_section(ms) || pfn_section_valid(ms, pfn); - rcu_read_unlock(); + rcu_read_unlock_sched();
return ret; }