Kees Cook <keescook@xxxxxxxxxxxx> writes:
> [+x86 folks]
>
> On Tue, Mar 4, 2014 at 7:57 AM, Linus Torvalds
> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>> On Mon, Mar 3, 2014 at 3:55 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>>> This got NAKed, please don't apply -- this patch works for x86 and
>>> ARM, but may cause problems for others:
>>
>> I'd much rather fix x86 and ARM, than worry about avr32.
>>
>> Priorities, people.
>>
>> Somebody who knows how "fix this properly" is supposed to work?
>
> I have not yet had a chance to more carefully examine the options, but
> AIUI, the problem is that (at least) the "per cpu" variables are
> neither absolute nor relative addresses from a relocation perspective.
> They're relative to the per cpu area, but the linker tools don't know
> about that state. So, I think, to fix this correctly, we need to teach
> the linker tools about this third state. This may also help
> arch/x86/tools/relocs, which is currently using a whitelist for
> mis-identified variables.
Well, __per_cpu_start points into a real section, within the discarded
init region. Makes me wonder why it's zero in /proc/kallsyms (it is on
my Ubuntu system here too).
... digging ...
Ah, the zero-based percpu patches, of course. Gah.
How's this? Did I break IA64?
===
kallsyms: make zero-based __per_cpu_start & __per_cpu_end absolute symbols.
Andy reported that x86-64 with CONFIG_RANDOMIZE_BASE has bogus values
for __per_cpu_start and __per_cpu_end in /proc/kallsyms:
Several kallsyms output in different boot states for comparison:
$ egrep '_(stext|_per_cpu_(start|end))' /root/kallsyms.nokaslr
0000000000000000 D __per_cpu_start
0000000000014280 D __per_cpu_end
ffffffff810001c8 T _stext
$ egrep '_(stext|_per_cpu_(start|end))' /root/kallsyms.kaslr1
000000001f200000 D __per_cpu_start
000000001f214280 D __per_cpu_end
ffffffffa02001c8 T _stext
$ egrep '_(stext|_per_cpu_(start|end))' /root/kallsyms.kaslr2
000000000d400000 D __per_cpu_start
000000000d414280 D __per_cpu_end
ffffffff8e4001c8 T _stext
$ egrep '_(stext|_per_cpu_(start|end))' /root/kallsyms.kaslr-fixed
0000000000000000 D __per_cpu_start
0000000000014280 D __per_cpu_end
ffffffffadc001c8 T _stext
x86-64 pretends that the per_cpu section is at address 0, and thus the
__per_cpu_start and __per_cpu_end resolve to 0 and <smallnum>.
kallsyms encodes the addresses in the kallsyms_addresses[] as relative
to _text: this means they get automatically relocated when the kernel
gets moved. But that's clearly wrong for the case where these are
fixed addresses.
/proc/kallsyms already handles absolute symbols, so just make
__per_cpu_start and __per_cpu_end absolute, and add them to the
inclusive list so kallsyms doesn't filter them out.
We make the change to absolute symbols in the PERCPU_VADDR linker
script macro, which is only used by x86 (when !CONFIG_X86_32) and
ia64, to place the per-cpu section at a specific address. This means
that using PERCPU_VADDR is wrong if you don't want an absolute
address, so we remove the comment about the vaddr arg being optional.
The comment on PERCPU_VADDR says __per_cpu_load is an absolute symbol,
but it isn't (and I don't think it should be). Delete that.
Reported-by: Andy Honig <ahonig@xxxxxxxxxx>
Signed-off-by: Rusty Russell <rusty@xxxxxxxxxxxxxxx>
diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index bc2121fa9132..c70e6242d1d6 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -674,6 +674,16 @@
*(.discard.*) \
}
+#define __PERCPU_INPUT(cacheline) \
+ *(.data..percpu..first) \
+ . = ALIGN(PAGE_SIZE); \
+ *(.data..percpu..page_aligned) \
+ . = ALIGN(cacheline); \
+ *(.data..percpu..readmostly) \
+ . = ALIGN(cacheline); \
+ *(.data..percpu) \
+ *(.data..percpu..shared_aligned) \
+
/**
* PERCPU_INPUT - the percpu input sections
* @cacheline: cacheline size
@@ -686,20 +697,13 @@
*/
#define PERCPU_INPUT(cacheline) \
VMLINUX_SYMBOL(__per_cpu_start) = .; \
- *(.data..percpu..first) \
- . = ALIGN(PAGE_SIZE); \
- *(.data..percpu..page_aligned) \
- . = ALIGN(cacheline); \
- *(.data..percpu..readmostly) \
- . = ALIGN(cacheline); \
- *(.data..percpu) \
- *(.data..percpu..shared_aligned) \
+ __PERCPU_INPUT(cacheline) \
VMLINUX_SYMBOL(__per_cpu_end) = .;
/**
* PERCPU_VADDR - define output section for percpu area
* @cacheline: cacheline size
- * @vaddr: explicit base address (optional)
+ * @vaddr: explicit base address
* @phdr: destination PHDR (optional)
*
* Macro which expands to output section for percpu area.
@@ -707,24 +711,25 @@
* @cacheline is used to align subsections to avoid false cacheline
* sharing between subsections for different purposes.
*
- * If @vaddr is not blank, it specifies explicit base address and all
- * percpu symbols will be offset from the given address. If blank,
- * @vaddr always equals @laddr + LOAD_OFFSET.
+ * @vaddr specifies explicit base address and all
+ * percpu symbols will be offset from the given address.
*
* @phdr defines the output PHDR to use if not blank. Be warned that
* output PHDR is sticky. If @phdr is specified, the next output
* section in the linker script will go there too. @phdr should have
* a leading colon.
*
- * Note that this macros defines __per_cpu_load as an absolute symbol.
- * If there is no need to put the percpu section at a predetermined
- * address, use PERCPU_SECTION.
+ * Note that this macros define __per_cpu_start and __per_cpu_end as
+ * absolute addresses. If there is no need to put the percpu section
+ * at a predetermined address, use PERCPU_SECTION.
*/
#define PERCPU_VADDR(cacheline, vaddr, phdr) \
VMLINUX_SYMBOL(__per_cpu_load) = .; \
.data..percpu vaddr : AT(VMLINUX_SYMBOL(__per_cpu_load) \
- LOAD_OFFSET) { \
- PERCPU_INPUT(cacheline) \
+ VMLINUX_SYMBOL(__per_cpu_start) = ABSOLUTE(.); \
+ __PERCPU_INPUT(cacheline) \
+ VMLINUX_SYMBOL(__per_cpu_end) = ABSOLUTE(.); \
} phdr \
. = VMLINUX_SYMBOL(__per_cpu_load) + SIZEOF(.data..percpu);
diff --git a/scripts/kallsyms.c b/scripts/kallsyms.c
index 10085de886fe..d6782918fe17 100644
--- a/scripts/kallsyms.c
+++ b/scripts/kallsyms.c
@@ -138,6 +138,7 @@ static int read_symbol(FILE *in, struct sym_entry *s)
if (strcmp(sym, "__kernel_syscall_via_break") &&
strcmp(sym, "__kernel_syscall_via_epc") &&
strcmp(sym, "__kernel_sigtramp") &&
+ strncmp(sym, "__per_cpu", strlen("__per_cpu")) &&
strcmp(sym, "__gp"))
return -1;
--
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