From: Andy Lutomirski <luto@xxxxxxxxxx> commit 4a13b0e3e10996b9aa0b45a764ecfe49f6fcd360 upstream. UNWIND_ESPFIX_STACK needs to read the GDT, and the GDT mapping that can be accessed via %fs is not mapped in the user pagetables. Use SGDT to find the cpu_entry_area mapping and read the espfix offset from that instead. Reported-and-tested-by: Borislav Petkov <bp@xxxxxxxxx> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Cc: <stable@xxxxxxxxxxxxxxx> Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- arch/x86/entry/entry_32.S | 21 ++++++++++++++++++--- 1 file changed, 18 insertions(+), 3 deletions(-) --- a/arch/x86/entry/entry_32.S +++ b/arch/x86/entry/entry_32.S @@ -315,7 +315,8 @@ .macro CHECK_AND_APPLY_ESPFIX #ifdef CONFIG_X86_ESPFIX32 -#define GDT_ESPFIX_SS PER_CPU_VAR(gdt_page) + (GDT_ENTRY_ESPFIX_SS * 8) +#define GDT_ESPFIX_OFFSET (GDT_ENTRY_ESPFIX_SS * 8) +#define GDT_ESPFIX_SS PER_CPU_VAR(gdt_page) + GDT_ESPFIX_OFFSET ALTERNATIVE "jmp .Lend_\@", "", X86_BUG_ESPFIX @@ -1056,12 +1057,26 @@ ENDPROC(entry_INT80_32) * We can't call C functions using the ESPFIX stack. This code reads * the high word of the segment base from the GDT and swiches to the * normal stack and adjusts ESP with the matching offset. + * + * We might be on user CR3 here, so percpu data is not mapped and we can't + * access the GDT through the percpu segment. Instead, use SGDT to find + * the cpu_entry_area alias of the GDT. */ #ifdef CONFIG_X86_ESPFIX32 /* fixup the stack */ - mov GDT_ESPFIX_SS + 4, %al /* bits 16..23 */ - mov GDT_ESPFIX_SS + 7, %ah /* bits 24..31 */ + pushl %ecx + subl $2*4, %esp + sgdt (%esp) + movl 2(%esp), %ecx /* GDT address */ + /* + * Careful: ECX is a linear pointer, so we need to force base + * zero. %cs is the only known-linear segment we have right now. + */ + mov %cs:GDT_ESPFIX_OFFSET + 4(%ecx), %al /* bits 16..23 */ + mov %cs:GDT_ESPFIX_OFFSET + 7(%ecx), %ah /* bits 24..31 */ shl $16, %eax + addl $2*4, %esp + popl %ecx addl %esp, %eax /* the adjusted stack pointer */ pushl $__KERNEL_DS pushl %eax
