From 8f5ca0b3195a9afff448004c2141d01a11f481da Mon Sep 17 00:00:00 2001 From: Benjamin Herrenschmidt Date: Wed, 19 Jul 2017 14:49:45 +1000 Subject: [PATCH] powerpc/mm: Cleanup check for stack expansion When hitting below a VM_GROWSDOWN vma (typically growing the stack), we check whether it's a valid stack-growing instruction and we check the distance to GPR1. This is largely open coded with lots of comments, so move it out to a helper. While at it, make store_update_sp a boolean. Signed-off-by: Benjamin Herrenschmidt Signed-off-by: Michael Ellerman --- arch/powerpc/mm/fault.c | 84 +++++++++++++++++++++++------------------ 1 file changed, 48 insertions(+), 36 deletions(-) diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c index e92448469d80..4e1f5e388020 100644 --- a/arch/powerpc/mm/fault.c +++ b/arch/powerpc/mm/fault.c @@ -71,15 +71,15 @@ static inline bool notify_page_fault(struct pt_regs *regs) * Check whether the instruction at regs->nip is a store using * an update addressing form which will update r1. */ -static int store_updates_sp(struct pt_regs *regs) +static bool store_updates_sp(struct pt_regs *regs) { unsigned int inst; if (get_user(inst, (unsigned int __user *)regs->nip)) - return 0; + return false; /* check for 1 in the rA field */ if (((inst >> 16) & 0x1f) != 1) - return 0; + return false; /* check major opcode */ switch (inst >> 26) { case 37: /* stwu */ @@ -87,7 +87,7 @@ static int store_updates_sp(struct pt_regs *regs) case 45: /* sthu */ case 53: /* stfsu */ case 55: /* stfdu */ - return 1; + return true; case 62: /* std or stdu */ return (inst & 3) == 1; case 31: @@ -99,10 +99,10 @@ static int store_updates_sp(struct pt_regs *regs) case 439: /* sthux */ case 695: /* stfsux */ case 759: /* stfdux */ - return 1; + return true; } } - return 0; + return false; } /* * do_page_fault error handling helpers @@ -222,6 +222,43 @@ static bool bad_kernel_fault(bool is_exec, unsigned long error_code, return is_exec || (address >= TASK_SIZE); } +static bool bad_stack_expansion(struct pt_regs *regs, unsigned long address, + struct vm_area_struct *vma, + bool store_update_sp) +{ + /* + * N.B. The POWER/Open ABI allows programs to access up to + * 288 bytes below the stack pointer. + * The kernel signal delivery code writes up to about 1.5kB + * below the stack pointer (r1) before decrementing it. + * The exec code can write slightly over 640kB to the stack + * before setting the user r1. Thus we allow the stack to + * expand to 1MB without further checks. + */ + if (address + 0x100000 < vma->vm_end) { + /* get user regs even if this fault is in kernel mode */ + struct pt_regs *uregs = current->thread.regs; + if (uregs == NULL) + return true; + + /* + * A user-mode access to an address a long way below + * the stack pointer is only valid if the instruction + * is one which would update the stack pointer to the + * address accessed if the instruction completed, + * i.e. either stwu rs,n(r1) or stwux rs,r1,rb + * (or the byte, halfword, float or double forms). + * + * If we don't check this then any write to the area + * between the last mapped region and the stack will + * expand the stack rather than segfaulting. + */ + if (address + 2048 < uregs->gpr[1] && !store_update_sp) + return true; + } + return false; +} + static bool access_error(bool is_write, bool is_exec, struct vm_area_struct *vma) { @@ -350,7 +387,7 @@ static int __do_page_fault(struct pt_regs *regs, unsigned long address, int is_user = user_mode(regs); int is_write = page_fault_is_write(error_code); int fault, major = 0; - int store_update_sp = 0; + bool store_update_sp = false; #ifdef CONFIG_PPC_ICSWX /* @@ -458,36 +495,11 @@ retry: if (unlikely(!(vma->vm_flags & VM_GROWSDOWN))) return bad_area(regs, address); - /* - * N.B. The POWER/Open ABI allows programs to access up to - * 288 bytes below the stack pointer. - * The kernel signal delivery code writes up to about 1.5kB - * below the stack pointer (r1) before decrementing it. - * The exec code can write slightly over 640kB to the stack - * before setting the user r1. Thus we allow the stack to - * expand to 1MB without further checks. - */ - if (address + 0x100000 < vma->vm_end) { - /* get user regs even if this fault is in kernel mode */ - struct pt_regs *uregs = current->thread.regs; - if (uregs == NULL) - return bad_area(regs, address); + /* The stack is being expanded, check if it's valid */ + if (unlikely(bad_stack_expansion(regs, address, vma, store_update_sp))) + return bad_area(regs, address); - /* - * A user-mode access to an address a long way below - * the stack pointer is only valid if the instruction - * is one which would update the stack pointer to the - * address accessed if the instruction completed, - * i.e. either stwu rs,n(r1) or stwux rs,r1,rb - * (or the byte, halfword, float or double forms). - * - * If we don't check this then any write to the area - * between the last mapped region and the stack will - * expand the stack rather than segfaulting. - */ - if (address + 2048 < uregs->gpr[1] && !store_update_sp) - return bad_area(regs, address); - } + /* Try to expand it */ if (unlikely(expand_stack(vma, address))) return bad_area(regs, address); -- 2.20.1