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author | Andy Lutomirski <luto@kernel.org> | 2017-03-18 22:17:24 -0700 |
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committer | Thomas Gleixner <tglx@linutronix.de> | 2017-03-19 12:14:35 +0100 |
commit | 5b781c7e317fcf9f74475dc82bfce2e359dfca13 (patch) | |
tree | 261067c7ce00f54a753fb3ae9a9cbac5e5b927cf /arch/x86/kernel/tls.c | |
parent | 2947ba054a4dabbd82848728d765346886050029 (diff) | |
download | linux-stable-5b781c7e317fcf9f74475dc82bfce2e359dfca13.tar.gz linux-stable-5b781c7e317fcf9f74475dc82bfce2e359dfca13.tar.bz2 linux-stable-5b781c7e317fcf9f74475dc82bfce2e359dfca13.zip |
x86/tls: Forcibly set the accessed bit in TLS segments
For mysterious historical reasons, struct user_desc doesn't indicate
whether segments are accessed. set_thread_area() has always programmed
segments as non-accessed, so the first write will set the accessed bit.
This will fault if the GDT is read-only.
Fix it by making TLS segments start out accessed.
If this ends up breaking something, we could, in principle, leave TLS
segments non-accessed and fix them up when we get the page fault. I'd be
surprised, though -- AFAIK all the nasty legacy segmented programs (DOSEMU,
Wine, things that run on DOSEMU and Wine, etc.) do their nasty segmented
things using the LDT and not the GDT. I assume this is mainly because old
OSes (Linux and otherwise) didn't historically provide APIs to do nasty
things in the GDT.
Fixes: 45fc8757d1d2 ("x86: Make the GDT remapping read-only on 64-bit")
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Thomas Garnier <thgarnie@google.com>
Link: http://lkml.kernel.org/r/62b7748542df0164af7e0a5231283b9b13858c45.1489900519.git.luto@kernel.org
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Diffstat (limited to 'arch/x86/kernel/tls.c')
-rw-r--r-- | arch/x86/kernel/tls.c | 11 |
1 files changed, 9 insertions, 2 deletions
diff --git a/arch/x86/kernel/tls.c b/arch/x86/kernel/tls.c index 6c8934406dc9..dcd699baea1b 100644 --- a/arch/x86/kernel/tls.c +++ b/arch/x86/kernel/tls.c @@ -92,10 +92,17 @@ static void set_tls_desc(struct task_struct *p, int idx, cpu = get_cpu(); while (n-- > 0) { - if (LDT_empty(info) || LDT_zero(info)) + if (LDT_empty(info) || LDT_zero(info)) { desc->a = desc->b = 0; - else + } else { fill_ldt(desc, info); + + /* + * Always set the accessed bit so that the CPU + * doesn't try to write to the (read-only) GDT. + */ + desc->type |= 1; + } ++info; ++desc; } |