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Date: Fri, 17 Feb 2017 11:00:39 +0000 From: James Morse <james.morse@....com> To: Stephen Boyd <stephen.boyd@...aro.org> CC: Catalin Marinas <catalin.marinas@....com>, Will Deacon <will.deacon@....com>, linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org, Laura Abbott <labbott@...hat.com>, Mark Rutland <mark.rutland@....com> Subject: Re: [PATCH v2] arm64: print a fault message when attempting to write RO memory Hi Stephen, On 17/02/17 01:19, Stephen Boyd wrote: > If a page is marked read only we should print out that fact, > instead of printing out that there was a page fault. Right now we > get a cryptic error message that something went wrong with an > unhandled fault, but we don't evaluate the esr to figure out that > it was a read/write permission fault. > > Instead of seeing: > > Unable to handle kernel paging request at virtual address ffff000008e460d8 > pgd = ffff800003504000 > [ffff000008e460d8] *pgd=0000000083473003, *pud=0000000083503003, *pmd=0000000000000000 > Internal error: Oops: 9600004f [#1] PREEMPT SMP > > we'll see: > > Unable to handle kernel write to read-only memory at virtual address ffff000008e760d8 > pgd = ffff80003d3de000 > [ffff000008e760d8] *pgd=0000000083472003, *pud=0000000083435003, *pmd=0000000000000000 > Internal error: Oops: 9600004f [#1] PREEMPT SMP This looks like a good idea.. > diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c > index 156169c6981b..8bd4e7f11c70 100644 > --- a/arch/arm64/mm/fault.c > +++ b/arch/arm64/mm/fault.c > /* > * The kernel tried to access some page that wasn't present. > */ > static void __do_kernel_fault(struct mm_struct *mm, unsigned long addr, > unsigned int esr, struct pt_regs *regs) > { > + const char *msg; > /* > * Are we prepared to handle this kernel fault? > * We are almost certainly not prepared to handle instruction faults. > @@ -177,9 +193,19 @@ static void __do_kernel_fault(struct mm_struct *mm, unsigned long addr, > * No handler, we'll have to terminate things with extreme prejudice. > */ > bust_spinlocks(1); > - pr_alert("Unable to handle kernel %s at virtual address %08lx\n", > - (addr < PAGE_SIZE) ? "NULL pointer dereference" : > - "paging request", addr); > + > + if (is_permission_fault(esr, regs)) { is_permission_fault() was previously guarded with a 'addr<USER_DS' check, this is because it assumes software-PAN is relevant. The corner case is when the kernel accesses TTBR1-mapped memory while software-PAN happens to have swivelled TTBR0. Translation faults will be matched by is_permission_fault(), but permission faults won't. Juggling is_permission_fault() to look something like: ---%<--- if (fsc_type == ESR_ELx_FSC_PERM) return true; if (addr < USER_DS && system_uses_ttbr0_pan()) return fsc_type == ESR_ELx_FSC_FAULT && (regs->pstate & PSR_PAN_BIT); return false; ---%<--- ... should fix this. > + if (esr & ESR_ELx_WNR) > + msg = "write to read-only memory"; > + else > + msg = "read from unreadable memory"; > + } else if (addr < PAGE_SIZE) > + msg = "NULL pointer dereference"; > + else > + msg = "paging request"; Nit: {} all the way down! > + > + pr_alert("Unable to handle kernel %s at virtual address %08lx\n", msg, > + addr); > > show_pte(mm, addr); > die("Oops", regs, esr); > @@ -269,21 +295,6 @@ static int __do_page_fault(struct mm_struct *mm, unsigned long addr, > return fault; > } > -static inline bool is_permission_fault(unsigned int esr, struct pt_regs *regs) > -{ > - unsigned int ec = ESR_ELx_EC(esr); > - unsigned int fsc_type = esr & ESR_ELx_FSC_TYPE; > - > - if (ec != ESR_ELx_EC_DABT_CUR && ec != ESR_ELx_EC_IABT_CUR) > - return false; > - > - if (system_uses_ttbr0_pan()) > - return fsc_type == ESR_ELx_FSC_FAULT && > - (regs->pstate & PSR_PAN_BIT); > - else > - return fsc_type == ESR_ELx_FSC_PERM; > -} Thanks! James
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