[Xen-devel] [PATCH for-4.9 v3 3/3] xen/livepatch: Don't crash on encountering STN_UNDEF relocations

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

A symndx of STN_UNDEF is special, and means a symbol value of 0.  While
legitimate in the ELF standard, its existance in a livepatch is questionable
at best.  Until a plausible usecase presents itself, reject such a relocation
with -EOPNOTSUPP.

Additionally, fix an off-by-one error while range checking symndx, and perform
a safety check on elf->sym[symndx].sym before derefencing it, to avoid
tripping over a NULL pointer when calculating val.

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

v3:
 * Fix off-by-one error
v2:
 * Reject STN_UNDEF with -EOPNOTSUPP
---
 xen/arch/arm/arm32/livepatch.c | 14 +++++++++++++-
 xen/arch/arm/arm64/livepatch.c | 14 +++++++++++++-
 xen/arch/x86/livepatch.c       | 14 +++++++++++++-
 3 files changed, 39 insertions(+), 3 deletions(-)

diff --git a/xen/arch/arm/arm32/livepatch.c b/xen/arch/arm/arm32/livepatch.c
index a328179..41378a5 100644
--- a/xen/arch/arm/arm32/livepatch.c
+++ b/xen/arch/arm/arm32/livepatch.c
@@ -254,12 +254,24 @@ int arch_livepatch_perform(struct livepatch_elf *elf,
             addend = get_addend(type, dest);
         }
 
-        if ( symndx > elf->nsym )
+        if ( symndx == STN_UNDEF )
+        {
+            dprintk(XENLOG_ERR, LIVEPATCH "%s: Encountered STN_UNDEF\n",
+                    elf->name);
+            return -EOPNOTSUPP;
+        }
+        else if ( symndx >= elf->nsym )
         {
             dprintk(XENLOG_ERR, LIVEPATCH "%s: Relative symbol wants symbol@%u 
which is past end!\n",
                     elf->name, symndx);
             return -EINVAL;
         }
+        else if ( !elf->sym[symndx].sym )
+        {
+            dprintk(XENLOG_ERR, LIVEPATCH "%s: No relative symbol@%u\n",
+                    elf->name, symndx);
+            return -EINVAL;
+        }
 
         val = elf->sym[symndx].sym->st_value; /* S */
 
diff --git a/xen/arch/arm/arm64/livepatch.c b/xen/arch/arm/arm64/livepatch.c
index 63929b1..2247b92 100644
--- a/xen/arch/arm/arm64/livepatch.c
+++ b/xen/arch/arm/arm64/livepatch.c
@@ -252,12 +252,24 @@ int arch_livepatch_perform_rela(struct livepatch_elf *elf,
         int ovf = 0;
         uint64_t val;
 
-        if ( symndx > elf->nsym )
+        if ( symndx == STN_UNDEF )
+        {
+            dprintk(XENLOG_ERR, LIVEPATCH "%s: Encountered STN_UNDEF\n",
+                    elf->name);
+            return -EOPNOTSUPP;
+        }
+        else if ( symndx >= elf->nsym )
         {
             dprintk(XENLOG_ERR, LIVEPATCH "%s: Relative relocation wants 
symbol@%u which is past end!\n",
                     elf->name, symndx);
             return -EINVAL;
         }
+        else if ( !elf->sym[symndx].sym )
+        {
+            dprintk(XENLOG_ERR, LIVEPATCH "%s: No relative symbol@%u\n",
+                    elf->name, symndx);
+            return -EINVAL;
+        }
 
         val = elf->sym[symndx].sym->st_value +  r->r_addend; /* S+A */
 
diff --git a/xen/arch/x86/livepatch.c b/xen/arch/x86/livepatch.c
index 7917610..406eb91 100644
--- a/xen/arch/x86/livepatch.c
+++ b/xen/arch/x86/livepatch.c
@@ -170,12 +170,24 @@ int arch_livepatch_perform_rela(struct livepatch_elf *elf,
         uint8_t *dest = base->load_addr + r->r_offset;
         uint64_t val;
 
-        if ( symndx > elf->nsym )
+        if ( symndx == STN_UNDEF )
+        {
+            dprintk(XENLOG_ERR, LIVEPATCH "%s: Encountered STN_UNDEF\n",
+                    elf->name);
+            return -EOPNOTSUPP;
+        }
+        else if ( symndx >= elf->nsym )
         {
             dprintk(XENLOG_ERR, LIVEPATCH "%s: Relative relocation wants 
symbol@%u which is past end!\n",
                     elf->name, symndx);
             return -EINVAL;
         }
+        else if ( !elf->sym[symndx].sym )
+        {
+            dprintk(XENLOG_ERR, LIVEPATCH "%s: No symbol@%u\n",
+                    elf->name, symndx);
+            return -EINVAL;
+        }
 
         val = r->r_addend + elf->sym[symndx].sym->st_value;
 
-- 
2.1.4


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