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| author | H. Peter Anvin <hpa@linux.intel.com> | 2014-03-16 15:31:54 -0700 |
|---|---|---|
| committer | Ben Hutchings <ben@decadent.org.uk> | 2014-04-30 16:23:26 +0100 |
| commit | a862b5c4076b1ba4dd6c87aebac478853dc6db47 (patch) | |
| tree | 93de2152f78d53ac2afdf71e83d1ed20f9d9005a /arch | |
| parent | 68289dd32489599ba7f351a3e9ee6bbb36757bc8 (diff) | |
| download | lwn-a862b5c4076b1ba4dd6c87aebac478853dc6db47.tar.gz lwn-a862b5c4076b1ba4dd6c87aebac478853dc6db47.zip | |
x86-64, modify_ldt: Ban 16-bit segments on 64-bit kernels
commit b3b42ac2cbae1f3cecbb6229964a4d48af31d382 upstream.
The IRET instruction, when returning to a 16-bit segment, only
restores the bottom 16 bits of the user space stack pointer. We have
a software workaround for that ("espfix") for the 32-bit kernel, but
it relies on a nonzero stack segment base which is not available in
32-bit mode.
Since 16-bit support is somewhat crippled anyway on a 64-bit kernel
(no V86 mode), and most (if not quite all) 64-bit processors support
virtualization for the users who really need it, simply reject
attempts at creating a 16-bit segment when running on top of a 64-bit
kernel.
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Link: http://lkml.kernel.org/n/tip-kicdm89kzw9lldryb1br9od0@git.kernel.org
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Diffstat (limited to 'arch')
| -rw-r--r-- | arch/x86/kernel/ldt.c | 11 |
1 files changed, 11 insertions, 0 deletions
diff --git a/arch/x86/kernel/ldt.c b/arch/x86/kernel/ldt.c index ea697263b373..4ac453114d7e 100644 --- a/arch/x86/kernel/ldt.c +++ b/arch/x86/kernel/ldt.c @@ -230,6 +230,17 @@ static int write_ldt(void __user *ptr, unsigned long bytecount, int oldmode) } } + /* + * On x86-64 we do not support 16-bit segments due to + * IRET leaking the high bits of the kernel stack address. + */ +#ifdef CONFIG_X86_64 + if (!ldt_info.seg_32bit) { + error = -EINVAL; + goto out_unlock; + } +#endif + fill_ldt(&ldt, &ldt_info); if (oldmode) ldt.avl = 0; |