From 3fb0fdb3bbe7aed495109b3296b06c2409734023 Mon Sep 17 00:00:00 2001 From: Andy Lutomirski Date: Sat, 13 Feb 2021 11:19:44 -0800 Subject: x86/stackprotector/32: Make the canary into a regular percpu variable 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 depends on whether stackprotector is enabled (this is 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_chk_guard, 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. (That name is special. We could use any symbol we want for the %fs-relative mode, but for CONFIG_SMP=n, gcc refuses to let us use any name other than __stack_chk_guard.) Forcibly disable stackprotector on older compilers that don't support the new options and turn the stack canary into a percpu variable. The "lazy GS" approach is now used for all 32-bit configurations. Also makes load_gs_index() work on 32-bit kernels. On 64-bit kernels, it loads the GS selector and updates the user GSBASE accordingly. (This is unchanged.) On 32-bit kernels, it loads the GS selector and updates GSBASE, which is now always the user base. This means that the overall effect is the same on 32-bit and 64-bit, which avoids some ifdeffery. [ bp: Massage commit message. ] Signed-off-by: Andy Lutomirski Signed-off-by: Borislav Petkov Link: https://lkml.kernel.org/r/c0ff7dba14041c7e5d1cae5d4df052f03759bef3.1613243844.git.luto@kernel.org --- arch/x86/Kconfig | 7 ++----- 1 file changed, 2 insertions(+), 5 deletions(-) (limited to 'arch/x86/Kconfig') diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig index 2792879d398e..10cc6199bf67 100644 --- a/arch/x86/Kconfig +++ b/arch/x86/Kconfig @@ -360,10 +360,6 @@ config X86_64_SMP def_bool y depends on X86_64 && SMP -config X86_32_LAZY_GS - def_bool y - depends on X86_32 && !STACKPROTECTOR - config ARCH_SUPPORTS_UPROBES def_bool y @@ -386,7 +382,8 @@ config CC_HAS_SANE_STACKPROTECTOR default $(success,$(srctree)/scripts/gcc-x86_32-has-stack-protector.sh $(CC)) help We have to make sure stack protector is unconditionally disabled if - the compiler produces broken code. + the compiler produces broken code or if it does not let us control + the segment on 32-bit kernels. menu "Processor type and features" -- cgit v1.2.3-59-g8ed1b