Re: [PATCH] module/ksymtab: use 64-bit relative reference for target symbol

[Ard Biesheuvel <ard.biesheuvel@arm.com>]

On 5/22/19 4:02 PM, Ard Biesheuvel wrote:
> The following commit
> 
>    7290d5809571 ("module: use relative references for __ksymtab entries")
> 
> updated the ksymtab handling of some KASLR capable architectures
> so that ksymtab entries are emitted as pairs of 32-bit relative
> references. This reduces the size of the entries, but more
> importantly, it gets rid of statically assigned absolute
> addresses, which require fixing up at boot time if the kernel
> is self relocating (which takes a 24 byte RELA entry for each
> member of the ksymtab struct).
> 
> Since ksymtab entries are always part of the same module as the
> symbol they export (or of the core kernel), it was assumed at the
> time that a 32-bit relative reference is always sufficient to
> capture the offset between a ksymtab entry and its target symbol.
> 
> Unfortunately, this is not always true: in the case of per-CPU
> variables, a per-CPU variable's base address (which usually differs
> from the actual address of any of its per-CPU copies) could be at
> an arbitrary offset from the ksymtab entry, and so it may be out
> of range for a 32-bit relative reference.
> 
> To make matters worse, we identified an issue in the arm64 module
> loader, where the overflow check applied to 32-bit place relative
> relocations uses the range that is specified in the AArch64 psABI,
> which is documented as having a 'blind spot' unless you explicitly
> narrow the range to match the signed vs unsigned interpretation of
> the relocation target [0]. This means that, in some cases, code
> importing those per-CPU variables from other modules may obtain a
> bogus reference and corrupt unrelated data.
> 
> So let's fix this issue by switching to a 64-bit place relative
> reference on 64-bit architectures for the ksymtab entry's target
> symbol. This uses a bit more memory in the entry itself, which is
> unfortunate, but it preserves the original intent, which was to
> make the value invariant under runtime relocation of the core
> kernel.
> 
> [0] https://lore.kernel.org/linux-arm-kernel/20190521125707.6115-1-ard.biesheuvel@arm.com
> 
> Cc: Jessica Yu <jeyu@kernel.org>
> Cc: <stable@vger.kernel.org> # v4.19+
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@arm.com>
> ---
> 
> Note that the name 'CONFIG_HAVE_ARCH_PREL32_RELOCATIONS' is no longer
> entirely accurate after this patch, so I will follow up with a patch
> to rename it to CONFIG_HAVE_ARCH_PREL_RELOCATIONS, but that doesn't
> require a backport to -stable so I have omitted it here.
> 
> Also note that for x86, this patch depends on b40a142b12b5 ("x86: Add
> support for 64-bit place relative relocations"), which will need to
> be backported to v4.19 (from v4.20) if this patch is applied to
> -stable.
> 

Unfortunately, this is not quite true. In addition to that patch, we 
need some changes to the x86 'relocs' tool so it can handle 64-bit 
relative references to per-CPU symbols, much like the support it has 
today for 32-bit relative references. I have coded it up, and will send 
it out as soon as I have confirmed that it works.


>   include/asm-generic/export.h |  9 +++++++--
>   include/linux/compiler.h     |  9 +++++++++
>   include/linux/export.h       | 14 ++++++++++----
>   kernel/module.c              |  2 +-
>   4 files changed, 27 insertions(+), 7 deletions(-)
> 
> diff --git a/include/asm-generic/export.h b/include/asm-generic/export.h
> index 294d6ae785d4..4d658b1e4707 100644
> --- a/include/asm-generic/export.h
> +++ b/include/asm-generic/export.h
> @@ -4,7 +4,7 @@
>   #ifndef KSYM_FUNC
>   #define KSYM_FUNC(x) x
>   #endif
> -#ifdef CONFIG_64BIT
> +#if defined(CONFIG_64BIT) && !defined(CONFIG_HAVE_ARCH_PREL32_RELOCATIONS)
>   #ifndef KSYM_ALIGN
>   #define KSYM_ALIGN 8
>   #endif
> @@ -19,7 +19,12 @@
>   
>   .macro __put, val, name
>   #ifdef CONFIG_HAVE_ARCH_PREL32_RELOCATIONS
> -	.long	\val - ., \name - .
> +#ifdef CONFIG_64BIT
> +	.quad	\val - .
> +#else
> +	.long	\val - .
> +#endif
> +	.long	\name - .
>   #elif defined(CONFIG_64BIT)
>   	.quad	\val, \name
>   #else
> diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> index 8aaf7cd026b0..33c65ebb7cfe 100644
> --- a/include/linux/compiler.h
> +++ b/include/linux/compiler.h
> @@ -305,6 +305,15 @@ static inline void *offset_to_ptr(const int *off)
>   	return (void *)((unsigned long)off + *off);
>   }
>   
> +/**
> + * loffset_to_ptr - convert a relative memory offset to an absolute pointer
> + * @off:	the address of the signed long offset value
> + */
> +static inline void *loffset_to_ptr(const long *off)
> +{
> +	return (void *)((unsigned long)off + *off);
> +}
> +
>   #endif /* __ASSEMBLY__ */
>   
>   /* Compile time object size, -1 for unknown */
> diff --git a/include/linux/export.h b/include/linux/export.h
> index fd8711ed9ac4..8f805b9f1c25 100644
> --- a/include/linux/export.h
> +++ b/include/linux/export.h
> @@ -43,6 +43,12 @@ extern struct module __this_module;
>   
>   #ifdef CONFIG_HAVE_ARCH_PREL32_RELOCATIONS
>   #include <linux/compiler.h>
> +#ifdef CONFIG_64BIT
> +#define __KSYMTAB_REL	".quad "
> +#else
> +#define __KSYMTAB_REL	".long "
> +#endif
> +
>   /*
>    * Emit the ksymtab entry as a pair of relative references: this reduces
>    * the size by half on 64-bit architectures, and eliminates the need for
> @@ -52,16 +58,16 @@ extern struct module __this_module;
>   #define __KSYMTAB_ENTRY(sym, sec)					\
>   	__ADDRESSABLE(sym)						\
>   	asm("	.section \"___ksymtab" sec "+" #sym "\", \"a\"	\n"	\
> -	    "	.balign	8					\n"	\
> +	    "	.balign	4					\n"	\
>   	    "__ksymtab_" #sym ":				\n"	\
> -	    "	.long	" #sym "- .				\n"	\
> +	    __KSYMTAB_REL #sym "- .				\n"	\
>   	    "	.long	__kstrtab_" #sym "- .			\n"	\
>   	    "	.previous					\n")
>   
>   struct kernel_symbol {
> -	int value_offset;
> +	long value_offset;
>   	int name_offset;
> -};
> +} __packed;
>   #else
>   #define __KSYMTAB_ENTRY(sym, sec)					\
>   	static const struct kernel_symbol __ksymtab_##sym		\
> diff --git a/kernel/module.c b/kernel/module.c
> index 6e6712b3aaf5..43efd46feeee 100644
> --- a/kernel/module.c
> +++ b/kernel/module.c
> @@ -541,7 +541,7 @@ static bool check_exported_symbol(const struct symsearch *syms,
>   static unsigned long kernel_symbol_value(const struct kernel_symbol *sym)
>   {
>   #ifdef CONFIG_HAVE_ARCH_PREL32_RELOCATIONS
> -	return (unsigned long)offset_to_ptr(&sym->value_offset);
> +	return (unsigned long)loffset_to_ptr(&sym->value_offset);
>   #else
>   	return sym->value;
>   #endif
> 

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