The following commit has been merged into the x86/urgent branch of tip: Commit-ID: cc801833a171163edb6385425349ba8903bd1b20 Gitweb: https://git.kernel.org/tip/cc801833a171163edb6385425349ba8903bd1b20 Author: Andy Lutomirski <l...@kernel.org> AuthorDate: Fri, 03 Jul 2020 10:02:57 -07:00 Committer: Thomas Gleixner <t...@linutronix.de> CommitterDate: Sat, 04 Jul 2020 19:47:26 +02:00
x86/ldt: Disable 16-bit segments on Xen PV Xen PV doesn't implement ESPFIX64, so they don't work right. Disable them. Also print a warning the first time anyone tries to use a 16-bit segment on a Xen PV guest that would otherwise allow it to help people diagnose this change in behavior. This gets us closer to having all x86 selftests pass on Xen PV. Signed-off-by: Andy Lutomirski <l...@kernel.org> Signed-off-by: Thomas Gleixner <t...@linutronix.de> Acked-by: Peter Zijlstra (Intel) <pet...@infradead.org> Link: https://lkml.kernel.org/r/92b2975459dfe5929ecf34c3896ad920bd9e3f2d.1593795633.git.l...@kernel.org --- arch/x86/kernel/ldt.c | 35 ++++++++++++++++++++++++++++++++++- 1 file changed, 34 insertions(+), 1 deletion(-) diff --git a/arch/x86/kernel/ldt.c b/arch/x86/kernel/ldt.c index 8748321..34e918a 100644 --- a/arch/x86/kernel/ldt.c +++ b/arch/x86/kernel/ldt.c @@ -29,6 +29,8 @@ #include <asm/mmu_context.h> #include <asm/pgtable_areas.h> +#include <xen/xen.h> + /* This is a multiple of PAGE_SIZE. */ #define LDT_SLOT_STRIDE (LDT_ENTRIES * LDT_ENTRY_SIZE) @@ -543,6 +545,37 @@ static int read_default_ldt(void __user *ptr, unsigned long bytecount) return bytecount; } +static bool allow_16bit_segments(void) +{ + if (!IS_ENABLED(CONFIG_X86_16BIT)) + return false; + +#ifdef CONFIG_XEN_PV + /* + * Xen PV does not implement ESPFIX64, which means that 16-bit + * segments will not work correctly. Until either Xen PV implements + * ESPFIX64 and can signal this fact to the guest or unless someone + * provides compelling evidence that allowing broken 16-bit segments + * is worthwhile, disallow 16-bit segments under Xen PV. + */ + if (xen_pv_domain()) { + static DEFINE_MUTEX(xen_warning); + static bool warned; + + mutex_lock(&xen_warning); + if (!warned) { + pr_info("Warning: 16-bit segments do not work correctly in a Xen PV guest\n"); + warned = true; + } + mutex_unlock(&xen_warning); + + return false; + } +#endif + + return true; +} + static int write_ldt(void __user *ptr, unsigned long bytecount, int oldmode) { struct mm_struct *mm = current->mm; @@ -574,7 +607,7 @@ static int write_ldt(void __user *ptr, unsigned long bytecount, int oldmode) /* The user wants to clear the entry. */ memset(&ldt, 0, sizeof(ldt)); } else { - if (!IS_ENABLED(CONFIG_X86_16BIT) && !ldt_info.seg_32bit) { + if (!ldt_info.seg_32bit && !allow_16bit_segments()) { error = -EINVAL; goto out; }
