From 8a8b56638bcac4e64cccc88bf95a0f9f4b19a2fb Mon Sep 17 00:00:00 2001 From: James Hogan Date: Fri, 28 Apr 2017 10:50:26 +0100 Subject: [PATCH] metag/uaccess: Fix access_ok() The __user_bad() macro used by access_ok() has a few corner cases noticed by Al Viro where it doesn't behave correctly: - The kernel range check has off by 1 errors which permit access to the first and last byte of the kernel mapped range. - The kernel range check ends at LINCORE_BASE rather than META_MEMORY_LIMIT, which is ineffective when the kernel is in global space (an extremely uncommon configuration). There are a couple of other shortcomings here too: - Access to the whole of the other address space is permitted (i.e. the global half of the address space when the kernel is in local space). This isn't ideal as it could theoretically still contain privileged mappings set up by the bootloader. - The size argument is unused, permitting user copies which start on valid pages at the end of the user address range and cross the boundary into the kernel address space (e.g. addr = 0x3ffffff0, size > 0x10). It isn't very convenient to add size checks when disallowing certain regions, and it seems far safer to be sure and explicit about what userland is able to access, so invert the logic to allow certain regions instead, and fix the off by 1 errors and missing size checks. This also allows the get_fs() == KERNEL_DS check to be more easily optimised into the user address range case. We now have 3 such allowed regions: - The user address range (incorporating the get_fs() == KERNEL_DS check). - NULL (some kernel code expects this to work, and we'll always catch the fault anyway). - The core code memory region. Fixes: 373cd784d0fc ("metag: Memory handling") Reported-by: Al Viro Signed-off-by: James Hogan Cc: linux-metag@vger.kernel.org Cc: stable@vger.kernel.org --- arch/metag/include/asm/uaccess.h | 40 +++++++++++++++++++------------- 1 file changed, 24 insertions(+), 16 deletions(-) diff --git a/arch/metag/include/asm/uaccess.h b/arch/metag/include/asm/uaccess.h index 469a2f1393d3..1e5f26d2dce8 100644 --- a/arch/metag/include/asm/uaccess.h +++ b/arch/metag/include/asm/uaccess.h @@ -28,24 +28,32 @@ #define segment_eq(a, b) ((a).seg == (b).seg) -#define __kernel_ok (segment_eq(get_fs(), KERNEL_DS)) -/* - * Explicitly allow NULL pointers here. Parts of the kernel such - * as readv/writev use access_ok to validate pointers, but want - * to allow NULL pointers for various reasons. NULL pointers are - * safe to allow through because the first page is not mappable on - * Meta. - * - * We also wish to avoid letting user code access the system area - * and the kernel half of the address space. - */ -#define __user_bad(addr, size) (((addr) > 0 && (addr) < META_MEMORY_BASE) || \ - ((addr) > PAGE_OFFSET && \ - (addr) < LINCORE_BASE)) - static inline int __access_ok(unsigned long addr, unsigned long size) { - return __kernel_ok || !__user_bad(addr, size); + /* + * Allow access to the user mapped memory area, but not the system area + * before it. The check extends to the top of the address space when + * kernel access is allowed (there's no real reason to user copy to the + * system area in any case). + */ + if (likely(addr >= META_MEMORY_BASE && addr < get_fs().seg && + size <= get_fs().seg - addr)) + return true; + /* + * Explicitly allow NULL pointers here. Parts of the kernel such + * as readv/writev use access_ok to validate pointers, but want + * to allow NULL pointers for various reasons. NULL pointers are + * safe to allow through because the first page is not mappable on + * Meta. + */ + if (!addr) + return true; + /* Allow access to core code memory area... */ + if (addr >= LINCORE_CODE_BASE && addr <= LINCORE_CODE_LIMIT && + size <= LINCORE_CODE_LIMIT + 1 - addr) + return true; + /* ... but no other areas. */ + return false; } #define access_ok(type, addr, size) __access_ok((unsigned long)(addr), \ -- 2.20.1