From: Will Deacon will@kernel.org
[ Upstream commit 834020366da9ab3fb87d1eb9a3160eb22dbed63a ]
Translation faults arising from cache maintenance instructions are rather unhelpfully reported with an FSR value where the WnR field is set to 1, indicating that the faulting access was a write. Since cache maintenance instructions on 32-bit ARM do not require any particular permissions, this can cause our private 'cacheflush' system call to fail spuriously if a translation fault is generated due to page aging when targetting a read-only VMA.
In this situation, we will return -EFAULT to userspace, although this is unfortunately suppressed by the popular '__builtin___clear_cache()' intrinsic provided by GCC, which returns void.
Although it's tempting to write this off as a userspace issue, we can actually do a little bit better on CPUs that support LPAE, even if the short-descriptor format is in use. On these CPUs, cache maintenance faults additionally set the CM field in the FSR, which we can use to suppress the write permission checks in the page fault handler and succeed in performing cache maintenance to read-only areas even in the presence of a translation fault.
Reported-by: Orion Hodson oth@google.com Signed-off-by: Will Deacon will@kernel.org Signed-off-by: Russell King rmk+kernel@armlinux.org.uk Signed-off-by: Sasha Levin sashal@kernel.org --- arch/arm/mm/fault.c | 4 ++-- arch/arm/mm/fault.h | 1 + 2 files changed, 3 insertions(+), 2 deletions(-)
diff --git a/arch/arm/mm/fault.c b/arch/arm/mm/fault.c index 0d20cd5940171..702a5542b11a8 100644 --- a/arch/arm/mm/fault.c +++ b/arch/arm/mm/fault.c @@ -211,7 +211,7 @@ static inline bool access_error(unsigned int fsr, struct vm_area_struct *vma) { unsigned int mask = VM_READ | VM_WRITE | VM_EXEC;
- if (fsr & FSR_WRITE) + if ((fsr & FSR_WRITE) && !(fsr & FSR_CM)) mask = VM_WRITE; if (fsr & FSR_LNX_PF) mask = VM_EXEC; @@ -281,7 +281,7 @@ do_page_fault(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
if (user_mode(regs)) flags |= FAULT_FLAG_USER; - if (fsr & FSR_WRITE) + if ((fsr & FSR_WRITE) && !(fsr & FSR_CM)) flags |= FAULT_FLAG_WRITE;
/* diff --git a/arch/arm/mm/fault.h b/arch/arm/mm/fault.h index 78830657cab3a..b014e57248044 100644 --- a/arch/arm/mm/fault.h +++ b/arch/arm/mm/fault.h @@ -5,6 +5,7 @@ * Fault status register encodings. We steal bit 31 for our own purposes. */ #define FSR_LNX_PF (1 << 31) +#define FSR_CM (1 << 13) #define FSR_WRITE (1 << 11) #define FSR_FS4 (1 << 10) #define FSR_FS3_0 (15)