On Mon, Oct 05, 2020 at 12:30:03PM -0700, Andy Lutomirski wrote: > On 32-bit kernels, the stackprotector canary is quite nasty -- it is > stored at %gs:(20), which is nasty because 32-bit kernels use %fs for > percpu storage. It's even nastier because it means that whether %gs > contains userspace state or kernel state while running kernel code > sepends on whether stackprotector is enabled (this is
depends > CONFIG_X86_32_LAZY_GS), and this setting radically changes the way > that segment selectors work. Supporting both variants is a > maintenance and testing mess. > > Merely rearranging so that percpu and the stack canary > share the same segment would be messy as the 32-bit percpu address > layout isn't currently compatible with putting a variable at a fixed > offset. > > Fortunately, GCC 8.1 added options that allow the stack canary to be > accessed as %fs:stack_canary, effectively turning it into an ordinary > percpu variable. This lets us get rid of all of the code to manage > the stack canary GDT descriptor and the CONFIG_X86_32_LAZY_GS mess. > > This patch forcibly disables stackprotector on older compilers that > don't support the new options and makes the stack canary into a > percpu variable. It'd be helpful to explicitly state that the so called "lazy GS" approach is now always used for i386. > Signed-off-by: Andy Lutomirski <l...@kernel.org> > --- ... > diff --git a/arch/x86/include/asm/suspend_32.h > b/arch/x86/include/asm/suspend_32.h > index fdbd9d7b7bca..eb872363ca82 100644 > --- a/arch/x86/include/asm/suspend_32.h > +++ b/arch/x86/include/asm/suspend_32.h > @@ -16,9 +16,7 @@ struct saved_context { > * On x86_32, all segment registers, with the possible exception of Is this still a "possible" exception, or is it now always an exception? > * gs, are saved at kernel entry in pt_regs. > */ > -#ifdef CONFIG_X86_32_LAZY_GS > u16 gs; > -#endif > unsigned long cr0, cr2, cr3, cr4; > u64 misc_enable; > bool misc_enable_saved; ... > diff --git a/arch/x86/kernel/tls.c b/arch/x86/kernel/tls.c > index 64a496a0687f..3c883e064242 100644 > --- a/arch/x86/kernel/tls.c > +++ b/arch/x86/kernel/tls.c > @@ -164,17 +164,11 @@ int do_set_thread_area(struct task_struct *p, int idx, > 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 > + load_gs_index(sel); Side topic, the "index" part of this is super confusing. I had to reread this entire patch after discovering load_gs_index is loadsegment on i386. Maybe also worth a shout out in the changelog? > } else { > #ifdef CONFIG_X86_64 > if (p->thread.fsindex == modified_sel)