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arch: Introduce post-init read-only memory
One of the easiest ways to protect the kernel from attack is to reduce the internal attack surface exposed when a "write" flaw is available. By making as much of the kernel read-only as possible, we reduce the attack surface. Many things are written to only during __init, and never changed again. These cannot be made "const" since the compiler will do the wrong thing (we do actually need to write to them). Instead, move these items into a memory region that will be made read-only during mark_rodata_ro() which happens after all kernel __init code has finished. This introduces __ro_after_init as a way to mark such memory, and adds some documentation about the existing __read_mostly marking. This improves the security of the Linux kernel by marking formerly read-write memory regions as read-only on a fully booted up system. Based on work by PaX Team and Brad Spengler. Signed-off-by: Kees Cook <keescook@chromium.org> Cc: Andy Lutomirski <luto@amacapital.net> Cc: Arnd Bergmann <arnd@arndb.de> Cc: Borislav Petkov <bp@alien8.de> Cc: Brad Spengler <spender@grsecurity.net> Cc: Brian Gerst <brgerst@gmail.com> Cc: David Brown <david.brown@linaro.org> Cc: Denys Vlasenko <dvlasenk@redhat.com> Cc: Emese Revfy <re.emese@gmail.com> Cc: H. Peter Anvin <hpa@zytor.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Mathias Krause <minipli@googlemail.com> Cc: Michael Ellerman <mpe@ellerman.id.au> Cc: PaX Team <pageexec@freemail.hu> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: kernel-hardening@lists.openwall.com Cc: linux-arch <linux-arch@vger.kernel.org> Link: http://lkml.kernel.org/r/1455748879-21872-5-git-send-email-keescook@chromium.org Signed-off-by: Ingo Molnar <mingo@kernel.org>
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@ -22,6 +22,9 @@
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#define __read_mostly __attribute__((__section__(".data..read_mostly")))
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/* Read-only memory is marked before mark_rodata_ro() is called. */
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#define __ro_after_init __read_mostly
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void parisc_cache_init(void); /* initializes cache-flushing */
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void disable_sr_hashing_asm(int); /* low level support for above */
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void disable_sr_hashing(void); /* turns off space register hashing */
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@ -256,6 +256,7 @@
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.rodata : AT(ADDR(.rodata) - LOAD_OFFSET) { \
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VMLINUX_SYMBOL(__start_rodata) = .; \
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*(.rodata) *(.rodata.*) \
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*(.data..ro_after_init) /* Read only after init */ \
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*(__vermagic) /* Kernel version magic */ \
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. = ALIGN(8); \
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VMLINUX_SYMBOL(__start___tracepoints_ptrs) = .; \
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@ -12,10 +12,24 @@
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#define SMP_CACHE_BYTES L1_CACHE_BYTES
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#endif
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/*
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* __read_mostly is used to keep rarely changing variables out of frequently
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* updated cachelines. If an architecture doesn't support it, ignore the
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* hint.
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*/
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#ifndef __read_mostly
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#define __read_mostly
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#endif
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/*
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* __ro_after_init is used to mark things that are read-only after init (i.e.
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* after mark_rodata_ro() has been called). These are effectively read-only,
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* but may get written to during init, so can't live in .rodata (via "const").
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*/
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#ifndef __ro_after_init
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#define __ro_after_init __attribute__((__section__(".data..ro_after_init")))
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#endif
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#ifndef ____cacheline_aligned
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#define ____cacheline_aligned __attribute__((__aligned__(SMP_CACHE_BYTES)))
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#endif
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