Re: [RFC PATCH v4 3/9] x86/cet/ibt: Add IBT legacy code bitmap allocation function
From: Eugene Syromiatnikov <hidden>
Date: 2018-10-03 19:56:53
Also in:
linux-arch, linux-doc, linux-mm, lkml
On Fri, Sep 21, 2018 at 08:05:47AM -0700, Yu-cheng Yu wrote:
quoted hunk ↗ jump to hunk
Indirect branch tracking provides an optional legacy code bitmap that indicates locations of non-IBT compatible code. When set, each bit in the bitmap represents a page in the linear address is legacy code. We allocate the bitmap only when the application requests it. Most applications do not need the bitmap. Signed-off-by: Yu-cheng Yu <redacted> --- arch/x86/kernel/cet.c | 45 +++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 45 insertions(+)diff --git a/arch/x86/kernel/cet.c b/arch/x86/kernel/cet.c index 6adfe795d692..a65d9745af08 100644 --- a/arch/x86/kernel/cet.c +++ b/arch/x86/kernel/cet.c@@ -314,3 +314,48 @@ void cet_disable_ibt(void) wrmsrl(MSR_IA32_U_CET, r); current->thread.cet.ibt_enabled = 0; } + +int cet_setup_ibt_bitmap(void) +{ + u64 r; + unsigned long bitmap; + unsigned long size; + + if (!cpu_feature_enabled(X86_FEATURE_IBT)) + return -EOPNOTSUPP; + + if (!current->thread.cet.ibt_bitmap_addr) { + /* + * Calculate size and put in thread header. + * may_expand_vm() needs this information. + */ + size = TASK_SIZE / PAGE_SIZE / BITS_PER_BYTE;
TASK_SIZE_MAX is likely needed here, as an application can easily switch between long an 32-bit protected mode. And then the case of a CPU that doesn't support 5LPT.
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+ current->thread.cet.ibt_bitmap_size = size; + bitmap = do_mmap_locked(0, size, PROT_READ | PROT_WRITE, + MAP_ANONYMOUS | MAP_PRIVATE, + VM_DONTDUMP); + + if (bitmap >= TASK_SIZE) {
Shouldn't bitmap be unmapped here?
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+ current->thread.cet.ibt_bitmap_size = 0; + return -ENOMEM; + } + + current->thread.cet.ibt_bitmap_addr = bitmap; + } + + /* + * Lower bits of MSR_IA32_CET_LEG_IW_EN are for IBT + * settings. Clear lower bits even bitmap is already + * page-aligned. + */ + bitmap = current->thread.cet.ibt_bitmap_addr; + bitmap &= PAGE_MASK;
In a hypothetical situation of bitmap & PAGE_MASK < bitmap that would lead to bitmap pointing to unmapped memory. A check that bitmap is sane would probably be better.
+ + /* + * Turn on IBT legacy bitmap. + */ + rdmsrl(MSR_IA32_U_CET, r); + r |= (MSR_IA32_CET_LEG_IW_EN | bitmap); + wrmsrl(MSR_IA32_U_CET, r); + return 0; +} -- 2.17.1