[kernel-hardening] Re: [PATCH v4 0/4] Introduce the initify gcc plugin

[Laura Abbott <labbott@redhat.com>]

On 12/16/2016 02:06 PM, Kees Cook wrote:
> Hi,
> 
> This is a continuation of Emese Revfy's initify plugin upstreaming. This
> is based on her v3, but updated with various fixes from her github tree.
> Additionally, I split off the printf attribute fixes and sent those
> separately.
> 
> This is the initify gcc plugin. The kernel already has a mechanism to
> free up code and data memory that is only used during kernel or module
> initialization.  This plugin will teach the compiler to find more such
> code and data that can be freed after initialization. It reduces memory
> usage.  The initify gcc plugin can be useful for embedded systems.
> 
> Originally it was a CII project supported by the Linux Foundation.
> 
> This plugin is the part of grsecurity/PaX.
> 
> The plugin supports all gcc versions from 4.5 to 7.0.
> 
> Changes on top of the PaX version (since March 6.). These are the important
> ones:
>  * move all local strings to init.rodata.str and exit.rodata.str
>    (not just __func__)
>  * report all initified strings and functions
>    (GCC_PLUGIN_INITIFY_VERBOSE config option)
>  * automatically discover init/exit functions and apply the __init or
>    __exit attributes on them
> 
> You can find more about the changes here:
> https://github.com/ephox-gcc-plugins/initify
> 
> This patch set is based on v4.9-rc2.
> 
> Some build statistics about the plugin:
> 
> On allyes config (amd64, gcc-6):
> * 8412 initified strings
> *  167 initified functions
> 
> On allmod config (i386, gcc-6):
> * 8597 initified strings
> *  159 initified functions
> 
> On allyes config (amd64, gcc-6):
> 
> section         vanilla                 vanilla + initify        change
> -----------------------------------------------------------------------
> .rodata         21746728 (0x14bd428)    21488680 (0x147e428)    -258048
> .init.data       1338376  (0x146c08)     1683016  (0x19ae48)    +344640
> .text           78270904 (0x4aa51b8)    78228280 (0x4a9ab38)     -42624
> .init.text       1184725  (0x1213d5)     1223257  (0x12aa59)     +38532
> .exit.data           104  (0x000068)       17760  (0x004560)     +17656
> .exit.text        174473  (0x02a989)      175763  (0x02ae93)      +1290
> 
>         FileSiz (vanilla)       FileSiz (vanilla + initify)      change
> ------------------------------------------------------------------------
> 00      102936576 (0x622b000)   102678528 (0x61ec000)           -258048
> 03       28680192 (0x1b5a000)    29081600 (0x1bbc000)           +401408
> 
> 00     .text .notes __ex_table .rodata __bug_table .pci_fixup .builtin_fw
>        .tracedata __ksymtab __ksymtab_gpl __ksymtab_strings __init_rodata
>        __param __modver
> 03     .init.text .altinstr_aux .init.data .x86_cpu_dev.init
>        .parainstructions .altinstructions .altinstr_replacement
>        .iommu_table .apicdrivers .exit.text .exit.data .smp_locks .bss .brk
> 
> 
> On defconfig (amd64, gcc-6):
> * 1957 initified strings
> *   29 initified functions
> 
> On defconfig (amd64, gcc-6):
> 
> section         vanilla                 vanilla + initify        change
> -----------------------------------------------------------------------
> .rodata         2524240 (0x268450)      2462800 (0x259450)      -61440
> .init.data       560256 (0x088c80)       644000 (0x09d3a0)      +83744
> .text           9377367 (0x8f1657)      9373079 (0x8f0597)       -4288
> .init.text       438586 (0x06b13a)       441828 (0x06bde4)       +3242
> .exit.data            0                     832 (0x000340)        +832
> .exit.text         8857 (0x002299)          8857 (0x002299)          0
> 
>         FileSiz (vanilla)       FileSiz (vanilla + initify)      change
> ------------------------------------------------------------------------
> 00      13398016 (0xcc7000)     13336576 (0xcb8000)             -61440
> 03       2203648 (0x21a000)      2293760 (0x230000)             +90112
> 
> 00     .text .notes __ex_table .rodata __bug_table .pci_fixup .builtin_fw
>        .tracedata __ksymtab __ksymtab_gpl __ksymtab_strings __init_rodata
>        __param __modver
> 03     .init.text .altinstr_aux .init.data .x86_cpu_dev.init
>        .parainstructions .altinstructions .altinstr_replacement
>        .iommu_table .apicdrivers .exit.text .exit.data .smp_locks .bss .brk
> 
> One thing of note is that this plugin triggers false positive warnings
> from the modpost section mismatch detector. Further work is needed to
> deal with this.
> 
> 
> Changed from v3:
>  * Refresh from Emese's latest version.
> 
> Changed from v2:
>  * Check all uses when walking a use-def chain.
>  * Check all uses of initialized local variables and initify them if they
>    have only nocapture uses. Previously only uses in call arguments
>    determined whether the initializer value could be initified.
>  * Handle the format gcc attribute from the plugin too.
>  * Verify nocapture parameters of calls. Track uses of these parameters
>    and verify that all uses are not captured. Verify only the nocapture
>    attribute (The format attribute should be verified too.).
>  * Fixed wrong indexing of function arguments.
>  * Fixed decl comparison. When comparing two decls the tree codes must be
>    the same.
>  * Search capture uses of the return value. Use negative nocapture
>    attribute parameter on a function argument to verify that the return
>    value is not captured.
>  * Stop the search for capture uses if there is a cast to integer type.
>  * Removed unnecessary duplication hook.
>  * Handle cloned functions with a changed argument list.
>  * Check visited tree nodes to avoid an infinite loop.
>  * Add a new initify plugin option: enable_init_to_exit_moves. Move a
>    function to the exit section if it is called by __init and __exit
>    functions too.
>  * Added plugin option to disable the search of capture uses in nocapture
>    functions. We must be able to disable verification of nocapture
>    functions because there is a lot of asm code in the str* and mem*
>    functions on i386.
>  * Added some more nocapture attributes.
>  * Added some more printf attributes.
>  * Added some unverified_nocapture attributes.
>  * Make is_kernel_rodata() nocapture.
>  * Added comment for the nocapture attribute from Kees.
> 
> Changes from v1:
>  * Removed unnecessary nocapture attributes from boot code
>    (Reported-by: PaX Team <pageexec@freemail.hu>)
>  * Removed nocapture attributes from functions that return
>    the marked parameter
>    (Reported-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>)
>  * Added nocapture attribute to strlen()
>  * Updated gcc-common.h from PaX
>  * Don't forcibly constify initified string types
>    this caused the size reduction of the .data section
>    (initify_plugin.c)
>  * Added the section mismatch problem in the commit message
> 

I gave this a spin on arm64 and it seems to boot up and run okay
with just the "select HAVE_GCC_PLUGIN_INITIFY_INIT_EXIT if GCC_PLUGINS"
added for arm64. The patches could probably use more review but
I think it would be good to just fold this in for arm64 for ease of
testing.

Thanks,
Laura