path: root/scripts/
diff options
authorAndy Lutomirski <>2021-02-13 11:19:44 -0800
committerBorislav Petkov <>2021-03-08 13:19:05 +0100
commit3fb0fdb3bbe7aed495109b3296b06c2409734023 (patch)
tree72e8821e31ab11816ab4bf26f7d41e915cb022cd /scripts/
parenta38fd8748464831584a19438cbb3082b5a2dab15 (diff)
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:
Diffstat (limited to 'scripts/')
1 files changed, 5 insertions, 1 deletions
diff --git a/scripts/ b/scripts/
index f5c119495254..825c75c5b715 100755
--- a/scripts/
+++ b/scripts/
@@ -1,4 +1,8 @@
# SPDX-License-Identifier: GPL-2.0
-echo "int foo(void) { char X[200]; return 3; }" | $* -S -x c -c -m32 -O0 -fstack-protector - -o - 2> /dev/null | grep -q "%gs"
+# This requires GCC 8.1 or better. Specifically, we require
+# -mstack-protector-guard-reg, added by
+echo "int foo(void) { char X[200]; return 3; }" | $* -S -x c -c -m32 -O0 -fstack-protector -mstack-protector-guard-reg=fs -mstack-protector-guard-symbol=__stack_chk_guard - -o - 2> /dev/null | grep -q "%fs"