From d47b50e7a111bb7a56fb1c974728b56209d7f515 Mon Sep 17 00:00:00 2001 From: Andy Lutomirski Date: Thu, 7 Apr 2016 17:31:45 -0700 Subject: [PATCH] x86/arch_prctl: Fix ARCH_GET_FS and ARCH_GET_GS ARCH_GET_FS and ARCH_GET_GS attempted to figure out the fsbase and gsbase respectively from saved thread state. This was wrong: fsbase and gsbase live in registers while a thread is running, not in memory. For reasons I can't fathom, the fsbase and gsbase code were different. Since neither was correct, I didn't try to figure out what the point of the difference was. Change it to simply read the MSRs. The code for reading the base for a remote thread is also completely wrong if the target thread uses its own descriptors (which is the case for all 32-bit threaded programs), but fixing that is a different story. Signed-off-by: Andy Lutomirski Reviewed-by: Borislav Petkov Cc: Andy Lutomirski Cc: Borislav Petkov Cc: Brian Gerst Cc: Denys Vlasenko Cc: H. Peter Anvin Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Rudolf Marek Cc: Thomas Gleixner Link: http://lkml.kernel.org/r/c6e7b507c72ca3bdbf6c7a8a3ceaa0334e873bd9.1460075211.git.luto@kernel.org Signed-off-by: Ingo Molnar --- arch/x86/kernel/process_64.c | 19 +++++++------------ 1 file changed, 7 insertions(+), 12 deletions(-) diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c index 6cbab31ac23a..c671b9b015c0 100644 --- a/arch/x86/kernel/process_64.c +++ b/arch/x86/kernel/process_64.c @@ -566,10 +566,10 @@ long do_arch_prctl(struct task_struct *task, int code, unsigned long addr) break; case ARCH_GET_FS: { unsigned long base; - if (task->thread.fsindex == FS_TLS_SEL) - base = read_32bit_tls(task, FS_TLS); - else if (doit) + if (doit) rdmsrl(MSR_FS_BASE, base); + else if (task->thread.fsindex == FS_TLS_SEL) + base = read_32bit_tls(task, FS_TLS); else base = task->thread.fs; ret = put_user(base, (unsigned long __user *)addr); @@ -577,16 +577,11 @@ long do_arch_prctl(struct task_struct *task, int code, unsigned long addr) } case ARCH_GET_GS: { unsigned long base; - unsigned gsindex; - if (task->thread.gsindex == GS_TLS_SEL) + if (doit) + rdmsrl(MSR_KERNEL_GS_BASE, base); + else if (task->thread.gsindex == GS_TLS_SEL) base = read_32bit_tls(task, GS_TLS); - else if (doit) { - savesegment(gs, gsindex); - if (gsindex) - rdmsrl(MSR_KERNEL_GS_BASE, base); - else - base = task->thread.gs; - } else + else base = task->thread.gs; ret = put_user(base, (unsigned long __user *)addr); break; -- 2.20.1