Messages in this thread Patch in this message | | | From | Andy Lutomirski <> | Subject | [PATCH resend] x86, tls: Don't validate lm in set_thread_area after all | Date | Wed, 17 Dec 2014 14:48:30 -0800 |
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It turns out that there's a lurking ABI issue. GCC, when compiling this in a 32-bit program:
struct user_desc desc = { .entry_number = idx, .base_addr = base, .limit = 0xfffff, .seg_32bit = 1, .contents = 0, /* Data, grow-up */ .read_exec_only = 0, .limit_in_pages = 1, .seg_not_present = 0, .useable = 0, };
will leave .lm uninitialized. This means that anything in the kernel that reads user_desc.lm for 32-bit tasks is unreliable.
Revert the .lm check in set_thread_area. The value never did anything in the first place.
Fixes: 0e58af4e1d21 x86/tls: Disallow unusual TLS segments Cc: stable@vger.kernel.org # Only if 0e58af4e1d21 is backported Signed-off-by: Andy Lutomirski <luto@amacapital.net> ---
I think that this got eaten by gmail's SMTP server. It showed up in my inbox, but it never made it to lkml.
arch/x86/include/uapi/asm/ldt.h | 7 +++++++ arch/x86/kernel/tls.c | 6 ------ 2 files changed, 7 insertions(+), 6 deletions(-)
diff --git a/arch/x86/include/uapi/asm/ldt.h b/arch/x86/include/uapi/asm/ldt.h index 46727eb37bfe..6e1aaf73852a 100644 --- a/arch/x86/include/uapi/asm/ldt.h +++ b/arch/x86/include/uapi/asm/ldt.h @@ -28,6 +28,13 @@ struct user_desc { unsigned int seg_not_present:1; unsigned int useable:1; #ifdef __x86_64__ + /* + * Because this bit is not present in 32-bit user code, user + * programs can pass uninitialized values here. Therefore, in + * any context in which a user_desc comes from a 32-bit program, + * the kernel must act as though lm == 0, regardless of the + * actual value. + */ unsigned int lm:1; #endif }; diff --git a/arch/x86/kernel/tls.c b/arch/x86/kernel/tls.c index 3e551eee87b9..4e942f31b1a7 100644 --- a/arch/x86/kernel/tls.c +++ b/arch/x86/kernel/tls.c @@ -55,12 +55,6 @@ static bool tls_desc_okay(const struct user_desc *info) if (info->seg_not_present) return false; -#ifdef CONFIG_X86_64 - /* The L bit makes no sense for data. */ - if (info->lm) - return false; -#endif - return true; } -- 2.1.0
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