From c9867f863e19dd07c6085b457f6775047924c3b5 Mon Sep 17 00:00:00 2001 From: Andy Lutomirski Date: Tue, 26 Apr 2016 12:23:30 -0700 Subject: [PATCH] x86/tls: Synchronize segment registers in set_thread_area() The current behavior of set_thread_area() when it modifies a segment that is currently loaded is a bit confused. If CS [1] or SS is modified, the change will take effect on return to userspace because CS and SS are fundamentally always reloaded on return to userspace. Similarly, on 32-bit kernels, if DS, ES, FS, or (depending on configuration) GS refers to a modified segment, the change will take effect immediately on return to user mode because the entry code reloads these registers. If set_thread_area() modifies DS, ES [2], FS, or GS on 64-bit kernels or GS on 32-bit lazy-GS [3] kernels, however, the segment registers will be left alone until something (most likely a context switch) causes them to be reloaded. This means that behavior visible to user space is inconsistent. If set_thread_area() is implicitly called via CLONE_SETTLS, then all segment registers will be reloaded before the thread starts because CLONE_SETTLS happens before the initial context switch into the newly created thread. Empirically, glibc requires the immediate reload on CLONE_SETTLS -- 32-bit glibc on my system does *not* manually reload GS when creating a new thread. Before enabling FSGSBASE, we need to figure out what the behavior will be, as FSGSBASE requires that we reconsider our behavior when, e.g., GS and GSBASE are out of sync in user mode. Given that we must preserve the existing behavior of CLONE_SETTLS, it makes sense to me that we simply extend similar behavior to all invocations of set_thread_area(). This patch explicitly updates any segment register referring to a segment that is targetted by set_thread_area(). If set_thread_area() deletes the segment, then the segment register will be nulled out. [1] This can't actually happen since 0e58af4e1d21 ("x86/tls: Disallow unusual TLS segments") but, if it did, this is how it would behave. [2] I strongly doubt that any existing non-malicious program loads a TLS segment into DS or ES on a 64-bit kernel because the context switch code was badly broken until recently, but that's not an excuse to leave the current code alone. [3] One way or another, that config option should to go away. Yuck! Signed-off-by: Andy Lutomirski Cc: Andy Lutomirski Cc: Borislav Petkov Cc: Brian Gerst Cc: Denys Vlasenko Cc: H. Peter Anvin Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Thomas Gleixner Link: http://lkml.kernel.org/r/27d119b0d396e9b82009e40dff8333a249038225.1461698311.git.luto@kernel.org Signed-off-by: Ingo Molnar --- arch/x86/kernel/tls.c | 42 ++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 42 insertions(+) diff --git a/arch/x86/kernel/tls.c b/arch/x86/kernel/tls.c index 7fc5e843f247..9692a5e9fdab 100644 --- a/arch/x86/kernel/tls.c +++ b/arch/x86/kernel/tls.c @@ -114,6 +114,7 @@ int do_set_thread_area(struct task_struct *p, int idx, int can_allocate) { struct user_desc info; + unsigned short __maybe_unused sel, modified_sel; if (copy_from_user(&info, u_info, sizeof(info))) return -EFAULT; @@ -141,6 +142,47 @@ int do_set_thread_area(struct task_struct *p, int idx, set_tls_desc(p, idx, &info, 1); + /* + * If DS, ES, FS, or GS points to the modified segment, forcibly + * refresh it. Only needed on x86_64 because x86_32 reloads them + * on return to user mode. + */ + modified_sel = (idx << 3) | 3; + + if (p == current) { +#ifdef CONFIG_X86_64 + savesegment(ds, sel); + if (sel == modified_sel) + loadsegment(ds, sel); + + savesegment(es, sel); + if (sel == modified_sel) + loadsegment(es, sel); + + savesegment(fs, sel); + if (sel == modified_sel) + loadsegment(fs, sel); + + savesegment(gs, sel); + if (sel == modified_sel) + load_gs_index(sel); +#endif + +#ifdef CONFIG_X86_32_LAZY_GS + savesegment(gs, sel); + if (sel == modified_sel) + loadsegment(gs, sel); +#endif + } else { +#ifdef CONFIG_X86_64 + if (p->thread.fsindex == modified_sel) + p->thread.fsbase = info.base_addr; + + if (p->thread.gsindex == modified_sel) + p->thread.gsbase = info.base_addr; +#endif + } + return 0; }