From 9db812dbb29d6244a9192e220deebdbc0080b38c Mon Sep 17 00:00:00 2001 From: "Eric W. Biederman" Date: Tue, 18 Sep 2018 01:23:35 +0200 Subject: [PATCH] signal/x86: Call force_sig_pkuerr from __bad_area_nosemaphore There is only one code path that can generate a pkuerr signal. That code path calls __bad_area_nosemaphore and can be dectected by testing if si_code == SEGV_PKUERR. It can be seen from inspection that all of the other tests in fill_sig_info_pkey are unnecessary. Therefore call force_sig_pkuerr directly from __bad_area_semaphore and remove fill_sig_info_pkey. At the same time move the comment above force_sig_info_pkey into bad_area_access_error, so that the documentation about pkey generation races is not lost. Reviewed-by: Thomas Gleixner Signed-off-by: "Eric W. Biederman" --- arch/x86/mm/fault.c | 76 +++++++++++++++++------------------------------------ 1 file changed, 24 insertions(+), 52 deletions(-) diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c index 35e7ab3..75e9dce 100644 --- a/arch/x86/mm/fault.c +++ b/arch/x86/mm/fault.c @@ -153,56 +153,6 @@ is_prefetch(struct pt_regs *regs, unsigned long error_code, unsigned long addr) return prefetch; } -/* - * A protection key fault means that the PKRU value did not allow - * access to some PTE. Userspace can figure out what PKRU was - * from the XSAVE state, and this function fills out a field in - * siginfo so userspace can discover which protection key was set - * on the PTE. - * - * If we get here, we know that the hardware signaled a X86_PF_PK - * fault and that there was a VMA once we got in the fault - * handler. It does *not* guarantee that the VMA we find here - * was the one that we faulted on. - * - * 1. T1 : mprotect_key(foo, PAGE_SIZE, pkey=4); - * 2. T1 : set PKRU to deny access to pkey=4, touches page - * 3. T1 : faults... - * 4. T2: mprotect_key(foo, PAGE_SIZE, pkey=5); - * 5. T1 : enters fault handler, takes mmap_sem, etc... - * 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_signo, int si_code, siginfo_t *info, - u32 *pkey) -{ - /* This is effectively an #ifdef */ - if (!boot_cpu_has(X86_FEATURE_OSPKE)) - return; - - /* Fault not from Protection Keys: nothing to do */ - if ((si_code != SEGV_PKUERR) || (si_signo != SIGSEGV)) - return; - /* - * force_sig_info_fault() is called from a number of - * contexts, some of which have a VMA and some of which - * do not. The X86_PF_PK handing happens after we have a - * valid VMA, so we should never reach this without a - * valid VMA. - */ - if (!pkey) { - WARN_ONCE(1, "PKU fault with no VMA passed in"); - info->si_pkey = 0; - return; - } - /* - * si_pkey should be thought of as a strong hint, but not - * absolutely guranteed to be 100% accurate because of - * the race explained above. - */ - info->si_pkey = *pkey; -} - static void force_sig_info_fault(int si_signo, int si_code, unsigned long address, struct task_struct *tsk, u32 *pkey) @@ -215,8 +165,6 @@ force_sig_info_fault(int si_signo, int si_code, unsigned long address, info.si_code = si_code; info.si_addr = (void __user *)address; - fill_sig_info_pkey(si_signo, si_code, &info, pkey); - force_sig_info(si_signo, &info, tsk); } @@ -884,6 +832,9 @@ __bad_area_nosemaphore(struct pt_regs *regs, unsigned long error_code, tsk->thread.error_code = error_code; tsk->thread.trap_nr = X86_TRAP_PF; + if (si_code == SEGV_PKUERR) + force_sig_pkuerr((void __user *)address, *pkey); + force_sig_info_fault(SIGSEGV, si_code, address, tsk, pkey); return; @@ -949,7 +900,28 @@ bad_area_access_error(struct pt_regs *regs, unsigned long error_code, * if pkeys are compiled out. */ if (bad_area_access_from_pkeys(error_code, vma)) { + /* + * A protection key fault means that the PKRU value did not allow + * access to some PTE. Userspace can figure out what PKRU was + * from the XSAVE state. This function captures the pkey from + * the vma and passes it to userspace so userspace can discover + * which protection key was set on the PTE. + * + * If we get here, we know that the hardware signaled a X86_PF_PK + * fault and that there was a VMA once we got in the fault + * handler. It does *not* guarantee that the VMA we find here + * was the one that we faulted on. + * + * 1. T1 : mprotect_key(foo, PAGE_SIZE, pkey=4); + * 2. T1 : set PKRU to deny access to pkey=4, touches page + * 3. T1 : faults... + * 4. T2: mprotect_key(foo, PAGE_SIZE, pkey=5); + * 5. T1 : enters fault handler, takes mmap_sem, etc... + * 6. T1 : reaches here, sees vma_pkey(vma)=5, when we really + * faulted on a pte with its pkey=4. + */ u32 pkey = vma_pkey(vma); + __bad_area(regs, error_code, address, &pkey, SEGV_PKUERR); } else { __bad_area(regs, error_code, address, NULL, SEGV_ACCERR); -- 2.7.4