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SubjectRe: [PATCH] x86/mm: Fix fault error path using unsafe vma pointer
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On 04/09/2017 10:32, Laurent Dufour wrote:
> The commit 7b2d0dbac489 ("x86/mm/pkeys: Pass VMA down in to fault signal
> generation code") pass down a vma pointer to the error path, but that is
> done once the mmap_sem is released when calling mm_fault_error() from
> __do_page_fault().
>
> This is dangerous as the pointed vma structure is no more safe to be used
> once the mmap_sem has been released. As only the protection key value is
> required in the error processing, we could just pass down this value.
>
> This patch fixes this by passing a pointer to a protection key value down
> to the fault signal generation code. The use of a pointer allows to keep
> the check generating a warning message in fill_sig_info_pkey() when the vma
> was not known. If the pointer is valid, the protection value can be
> accessed by deferencing the pointer.
>
> Fixes: 7b2d0dbac489 ("x86/mm/pkeys: Pass VMA down in to fault signal
> generation code")
> Cc: Dave Hansen <dave.hansen@linux.intel.com>
> Signed-off-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>

Hi all, any interest to this ?

Cheers,
Laurent.

> ---
> arch/x86/mm/fault.c | 47 ++++++++++++++++++++++++-----------------------
> 1 file changed, 24 insertions(+), 23 deletions(-)
>
> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index 2a1fa10c6a98..c18e737c5f9b 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -192,8 +192,7 @@ is_prefetch(struct pt_regs *regs, unsigned long error_code, unsigned long addr)
> * 6. T1 : reaches here, sees vma_pkey(vma)=5, when we really
> * faulted on a pte with its pkey=4.
> */
> -static void fill_sig_info_pkey(int si_code, siginfo_t *info,
> - struct vm_area_struct *vma)
> +static void fill_sig_info_pkey(int si_code, siginfo_t *info, int *pkey)
> {
> /* This is effectively an #ifdef */
> if (!boot_cpu_has(X86_FEATURE_OSPKE))
> @@ -209,7 +208,7 @@ static void fill_sig_info_pkey(int si_code, siginfo_t *info,
> * valid VMA, so we should never reach this without a
> * valid VMA.
> */
> - if (!vma) {
> + if (!pkey) {
> WARN_ONCE(1, "PKU fault with no VMA passed in");
> info->si_pkey = 0;
> return;
> @@ -219,13 +218,12 @@ static void fill_sig_info_pkey(int si_code, siginfo_t *info,
> * absolutely guranteed to be 100% accurate because of
> * the race explained above.
> */
> - info->si_pkey = vma_pkey(vma);
> + info->si_pkey = *pkey;
> }
>
> static void
> force_sig_info_fault(int si_signo, int si_code, unsigned long address,
> - struct task_struct *tsk, struct vm_area_struct *vma,
> - int fault)
> + struct task_struct *tsk, int *pkey, int fault)
> {
> unsigned lsb = 0;
> siginfo_t info;
> @@ -240,7 +238,7 @@ force_sig_info_fault(int si_signo, int si_code, unsigned long address,
> lsb = PAGE_SHIFT;
> info.si_addr_lsb = lsb;
>
> - fill_sig_info_pkey(si_code, &info, vma);
> + fill_sig_info_pkey(si_code, &info, pkey);
>
> force_sig_info(si_signo, &info, tsk);
> }
> @@ -758,8 +756,6 @@ no_context(struct pt_regs *regs, unsigned long error_code,
> struct task_struct *tsk = current;
> unsigned long flags;
> int sig;
> - /* No context means no VMA to pass down */
> - struct vm_area_struct *vma = NULL;
>
> /* Are we prepared to handle this kernel fault? */
> if (fixup_exception(regs, X86_TRAP_PF)) {
> @@ -784,7 +780,7 @@ no_context(struct pt_regs *regs, unsigned long error_code,
>
> /* XXX: hwpoison faults will set the wrong code. */
> force_sig_info_fault(signal, si_code, address,
> - tsk, vma, 0);
> + tsk, NULL, 0);
> }
>
> /*
> @@ -893,8 +889,7 @@ show_signal_msg(struct pt_regs *regs, unsigned long error_code,
>
> static void
> __bad_area_nosemaphore(struct pt_regs *regs, unsigned long error_code,
> - unsigned long address, struct vm_area_struct *vma,
> - int si_code)
> + unsigned long address, int *pkey, int si_code)
> {
> struct task_struct *tsk = current;
>
> @@ -942,7 +937,7 @@ __bad_area_nosemaphore(struct pt_regs *regs, unsigned long error_code,
> tsk->thread.error_code = error_code;
> tsk->thread.trap_nr = X86_TRAP_PF;
>
> - force_sig_info_fault(SIGSEGV, si_code, address, tsk, vma, 0);
> + force_sig_info_fault(SIGSEGV, si_code, address, tsk, pkey, 0);
>
> return;
> }
> @@ -955,9 +950,9 @@ __bad_area_nosemaphore(struct pt_regs *regs, unsigned long error_code,
>
> static noinline void
> bad_area_nosemaphore(struct pt_regs *regs, unsigned long error_code,
> - unsigned long address, struct vm_area_struct *vma)
> + unsigned long address, int *pkey)
> {
> - __bad_area_nosemaphore(regs, error_code, address, vma, SEGV_MAPERR);
> + __bad_area_nosemaphore(regs, error_code, address, pkey, SEGV_MAPERR);
> }
>
> static void
> @@ -965,6 +960,10 @@ __bad_area(struct pt_regs *regs, unsigned long error_code,
> unsigned long address, struct vm_area_struct *vma, int si_code)
> {
> struct mm_struct *mm = current->mm;
> + int pkey;
> +
> + if (vma)
> + pkey = vma_pkey(vma);
>
> /*
> * Something tried to access memory that isn't in our memory map..
> @@ -972,7 +971,8 @@ __bad_area(struct pt_regs *regs, unsigned long error_code,
> */
> up_read(&mm->mmap_sem);
>
> - __bad_area_nosemaphore(regs, error_code, address, vma, si_code);
> + __bad_area_nosemaphore(regs, error_code, address,
> + (vma) ? &pkey : NULL, si_code);
> }
>
> static noinline void
> @@ -1015,7 +1015,7 @@ bad_area_access_error(struct pt_regs *regs, unsigned long error_code,
>
> static void
> do_sigbus(struct pt_regs *regs, unsigned long error_code, unsigned long address,
> - struct vm_area_struct *vma, unsigned int fault)
> + int *pkey, unsigned int fault)
> {
> struct task_struct *tsk = current;
> int code = BUS_ADRERR;
> @@ -1042,13 +1042,12 @@ do_sigbus(struct pt_regs *regs, unsigned long error_code, unsigned long address,
> code = BUS_MCEERR_AR;
> }
> #endif
> - force_sig_info_fault(SIGBUS, code, address, tsk, vma, fault);
> + force_sig_info_fault(SIGBUS, code, address, tsk, pkey, fault);
> }
>
> static noinline void
> mm_fault_error(struct pt_regs *regs, unsigned long error_code,
> - unsigned long address, struct vm_area_struct *vma,
> - unsigned int fault)
> + unsigned long address, int *pkey, unsigned int fault)
> {
> if (fatal_signal_pending(current) && !(error_code & PF_USER)) {
> no_context(regs, error_code, address, 0, 0);
> @@ -1072,9 +1071,9 @@ mm_fault_error(struct pt_regs *regs, unsigned long error_code,
> } else {
> if (fault & (VM_FAULT_SIGBUS|VM_FAULT_HWPOISON|
> VM_FAULT_HWPOISON_LARGE))
> - do_sigbus(regs, error_code, address, vma, fault);
> + do_sigbus(regs, error_code, address, pkey, fault);
> else if (fault & VM_FAULT_SIGSEGV)
> - bad_area_nosemaphore(regs, error_code, address, vma);
> + bad_area_nosemaphore(regs, error_code, address, pkey);
> else
> BUG();
> }
> @@ -1268,6 +1267,7 @@ __do_page_fault(struct pt_regs *regs, unsigned long error_code,
> struct mm_struct *mm;
> int fault, major = 0;
> unsigned int flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
> + int pkey;
>
> tsk = current;
> mm = tsk->mm;
> @@ -1468,9 +1468,10 @@ __do_page_fault(struct pt_regs *regs, unsigned long error_code,
> return;
> }
>
> + pkey = vma_pkey(vma);
> up_read(&mm->mmap_sem);
> if (unlikely(fault & VM_FAULT_ERROR)) {
> - mm_fault_error(regs, error_code, address, vma, fault);
> + mm_fault_error(regs, error_code, address, &pkey, fault);
> return;
> }
>
>

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