diff options
| author | Sean Christopherson | 2022-05-26 21:08:11 +0000 |
|---|---|---|
| committer | Paolo Bonzini | 2022-06-10 10:01:29 -0400 |
| commit | dfe21e6bc05af433308bc1842da28a8fe28faaa4 (patch) | |
| tree | bacc1a7663d5f93a8023d18e124da34823f0ef34 /arch | |
| parent | 61d9c412d0416aa1f7914a732d424a9e8ff24c36 (diff) | |
KVM: x86: Harden _regs accesses to guard against buggy input
WARN and truncate the incoming GPR number/index when reading/writing GPRs
in the emulator to guard against KVM bugs, e.g. to avoid out-of-bounds
accesses to ctxt->_regs[] if KVM generates a bogus index. Truncate the
index instead of returning e.g. zero, as reg_write() returns a pointer
to the register, i.e. returning zero would result in a NULL pointer
dereference. KVM could also force the index to any arbitrary GPR, but
that's no better or worse, just different.
Open code the restriction to 16 registers; RIP is handled via _eip and
should never be accessed through reg_read() or reg_write(). See the
comments above the declarations of reg_read() and reg_write(), and the
behavior of writeback_registers(). The horrific open coded mess will be
cleaned up in a future commit.
There are no such bugs known to exist in the emulator, but determining
that KVM is bug-free is not at all simple and requires a deep dive into
the emulator. The code is so convoluted that GCC-12 with the recently
enable -Warray-bounds spits out a false-positive due to a GCC bug:
arch/x86/kvm/emulate.c:254:27: warning: array subscript 32 is above array
bounds of 'long unsigned int[17]' [-Warray-bounds]
254 | return ctxt->_regs[nr];
| ~~~~~~~~~~~^~~~
In file included from arch/x86/kvm/emulate.c:23:
arch/x86/kvm/kvm_emulate.h: In function 'reg_rmw':
arch/x86/kvm/kvm_emulate.h:366:23: note: while referencing '_regs'
366 | unsigned long _regs[NR_VCPU_REGS];
| ^~~~~
Link: https://lore.kernel.org/all/YofQlBrlx18J7h9Y@google.com
Link: https://bugzilla.kernel.org/show_bug.cgi?id=216026
Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105679
Reported-and-tested-by: Robert Dinse <nanook@eskimo.com>
Reported-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220526210817.3428868-3-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Diffstat (limited to 'arch')
| -rw-r--r-- | arch/x86/kvm/emulate.c | 6 |
1 files changed, 6 insertions, 0 deletions
diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c index 7226a127ccb4..c58366ae4da2 100644 --- a/arch/x86/kvm/emulate.c +++ b/arch/x86/kvm/emulate.c @@ -247,6 +247,9 @@ enum x86_transfer_type { static ulong reg_read(struct x86_emulate_ctxt *ctxt, unsigned nr) { + if (WARN_ON_ONCE(nr >= 16)) + nr &= 16 - 1; + if (!(ctxt->regs_valid & (1 << nr))) { ctxt->regs_valid |= 1 << nr; ctxt->_regs[nr] = ctxt->ops->read_gpr(ctxt, nr); @@ -256,6 +259,9 @@ static ulong reg_read(struct x86_emulate_ctxt *ctxt, unsigned nr) static ulong *reg_write(struct x86_emulate_ctxt *ctxt, unsigned nr) { + if (WARN_ON_ONCE(nr >= 16)) + nr &= 16 - 1; + ctxt->regs_valid |= 1 << nr; ctxt->regs_dirty |= 1 << nr; return &ctxt->_regs[nr]; |