From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Content-Type: text/plain; charset="UTF-8" Content-Disposition: inline Content-Transfer-Encoding: 8bit MIME-Version: 1.0 From: Ben Hutchings To: linux-kernel@vger.kernel.org, stable@vger.kernel.org CC: akpm@linux-foundation.org, "Linus Torvalds" , "Thomas Gleixner" , "Ingo Molnar" , "Andy Lutomirski" Date: Tue, 17 Feb 2015 01:46:53 +0000 Message-ID: Subject: [PATCH 3.2 063/152] x86/tls: Don't validate lm in set_thread_area() after all In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org List-ID: 3.2.67-rc1 review patch. If anyone has any objections, please let me know. ------------------ From: Andy Lutomirski commit 3fb2f4237bb452eb4e98f6a5dbd5a445b4fed9d0 upstream. It turns out that there's a lurking ABI issue. GCC, when compiling this in a 32-bit program: struct user_desc desc = { .entry_number = idx, .base_addr = base, .limit = 0xfffff, .seg_32bit = 1, .contents = 0, /* Data, grow-up */ .read_exec_only = 0, .limit_in_pages = 1, .seg_not_present = 0, .useable = 0, }; will leave .lm uninitialized. This means that anything in the kernel that reads user_desc.lm for 32-bit tasks is unreliable. Revert the .lm check in set_thread_area(). The value never did anything in the first place. Fixes: 0e58af4e1d21 ("x86/tls: Disallow unusual TLS segments") Signed-off-by: Andy Lutomirski Acked-by: Thomas Gleixner Cc: Linus Torvalds Link: http://lkml.kernel.org/r/d7875b60e28c512f6a6fc0baf5714d58e7eaadbb.1418856405.git.luto@amacapital.net Signed-off-by: Ingo Molnar [bwh: Backported to 3.2: adjust filename] Signed-off-by: Ben Hutchings --- arch/x86/include/asm/ldt.h | 7 +++++++ arch/x86/kernel/tls.c | 6 ------ 2 files changed, 7 insertions(+), 6 deletions(-) --- a/arch/x86/include/asm/ldt.h +++ b/arch/x86/include/asm/ldt.h @@ -28,6 +28,13 @@ struct user_desc { unsigned int seg_not_present:1; unsigned int useable:1; #ifdef __x86_64__ + /* + * Because this bit is not present in 32-bit user code, user + * programs can pass uninitialized values here. Therefore, in + * any context in which a user_desc comes from a 32-bit program, + * the kernel must act as though lm == 0, regardless of the + * actual value. + */ unsigned int lm:1; #endif }; --- a/arch/x86/kernel/tls.c +++ b/arch/x86/kernel/tls.c @@ -56,12 +56,6 @@ static bool tls_desc_okay(const struct u if (info->seg_not_present) return false; -#ifdef CONFIG_X86_64 - /* The L bit makes no sense for data. */ - if (info->lm) - return false; -#endif - return true; }