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Re: [Xen-devel] [PATCH v5 08/16] livepatch/arm/x86: Check payload for for unwelcomed symbols.

Hi Konrad,

On 21/09/16 18:32, Konrad Rzeszutek Wilk wrote:
Certain platforms, such as ARM [32|64] add extra mapping symbols
such as $x (for ARM64 instructions), or more interesting to
this patch: $t (for Thumb instructions). These symbols are suppose

nit: s/are suppose to/are supposed to/

to help the final linker to make any adjustments (such as
add an veneer). But more importantly - we do not compile Xen
with any Thumb instructions (which are variable length) - and
if we find these mapping symbols we should disallow such payload.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
Cc: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
Cc: Julien Grall <julien.grall@xxxxxxx
Cc: Jan Beulich <jbeulich@xxxxxxxx>
Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

v3: New submission.
    Use &sym[i] instead of sym (as that will always be NULL).
v4: Use bool instead of int for return
    Update comment in common code about ARM odd symbols.
    s/_check/_deny/ to make it more clear.
v5: Also check for $t.* wildcard.
    Use Julien's variant where we roll the [2] check in the return.
 xen/arch/arm/livepatch.c    | 16 ++++++++++++++++
 xen/arch/x86/livepatch.c    |  7 +++++++
 xen/common/livepatch_elf.c  |  7 +++++++
 xen/include/xen/livepatch.h |  2 ++
 4 files changed, 32 insertions(+)

diff --git a/xen/arch/arm/livepatch.c b/xen/arch/arm/livepatch.c
index 9959315..5a99ab5 100644
--- a/xen/arch/arm/livepatch.c
+++ b/xen/arch/arm/livepatch.c
@@ -117,6 +117,22 @@ bool arch_livepatch_symbol_ok(const struct livepatch_elf 
     return true;

+bool arch_livepatch_symbol_deny(const struct livepatch_elf *elf,
+                                const struct livepatch_elf_sym *sym)
+#ifdef CONFIG_ARM_32

Out of interest, is there any particular reason to use #ifdef rather than adding the function in {arm32/arm64}/livepatch.c?

+    /*
+     * Xen does not use Thumb instructions - and we should not see any of
+     * them. If we do, abort.
+     */
+    if ( sym->name && sym->name[0] == '$' && sym->name[1] == 't' )
+    {
+        return ( !sym->name[2] || sym->name[2] == '.' );
+    }

NIT: You could drop { } here.

+    return false;

Regardless the answer to my question:

Reviewed-by: Julien Grall <julien.grall@xxxxxxx>


Julien Grall

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