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

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

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
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 
*elf,
     return true;
 }
 
+bool arch_livepatch_symbol_deny(const struct livepatch_elf *elf,
+                                const struct livepatch_elf_sym *sym)
+{
+#ifdef CONFIG_ARM_32
+    /*
+     * 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] == '.' );
+    }
+#endif
+    return false;
+}
+
 int arch_livepatch_perform_rel(struct livepatch_elf *elf,
                                const struct livepatch_elf_sec *base,
                                const struct livepatch_elf_sec *rela)
diff --git a/xen/arch/x86/livepatch.c b/xen/arch/x86/livepatch.c
index 7a369a0..9663ef6 100644
--- a/xen/arch/x86/livepatch.c
+++ b/xen/arch/x86/livepatch.c
@@ -131,6 +131,13 @@ bool arch_livepatch_symbol_ok(const struct livepatch_elf 
*elf,
     return true;
 }
 
+bool arch_livepatch_symbol_deny(const struct livepatch_elf *elf,
+                                const struct livepatch_elf_sym *sym)
+{
+    /* No special checks on x86. */
+    return false;
+}
+
 int arch_livepatch_perform_rel(struct livepatch_elf *elf,
                                const struct livepatch_elf_sec *base,
                                const struct livepatch_elf_sec *rela)
diff --git a/xen/common/livepatch_elf.c b/xen/common/livepatch_elf.c
index f46990e..ec74beb 100644
--- a/xen/common/livepatch_elf.c
+++ b/xen/common/livepatch_elf.c
@@ -251,6 +251,13 @@ static int elf_get_sym(struct livepatch_elf *elf, const 
void *data)
 
         sym[i].sym = s;
         sym[i].name = strtab_sec->data + delta;
+        /* e.g. On ARM we should NEVER see $t* symbols. */
+        if ( arch_livepatch_symbol_deny(elf, &sym[i]) )
+        {
+            dprintk(XENLOG_ERR, LIVEPATCH "%s: Symbol '%s' should not be in 
payload!\n",
+                    elf->name, sym[i].name);
+            return -EINVAL;
+        }
     }
     elf->nsym = nsym;
 
diff --git a/xen/include/xen/livepatch.h b/xen/include/xen/livepatch.h
index e8c67d6..98ec012 100644
--- a/xen/include/xen/livepatch.h
+++ b/xen/include/xen/livepatch.h
@@ -50,6 +50,8 @@ bool_t is_patch(const void *addr);
 int arch_livepatch_verify_elf(const struct livepatch_elf *elf);
 bool arch_livepatch_symbol_ok(const struct livepatch_elf *elf,
                               const struct livepatch_elf_sym *sym);
+bool arch_livepatch_symbol_deny(const struct livepatch_elf *elf,
+                                const struct livepatch_elf_sym *sym);
 int arch_livepatch_perform_rel(struct livepatch_elf *elf,
                                const struct livepatch_elf_sec *base,
                                const struct livepatch_elf_sec *rela);
-- 
2.4.11


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