| Message ID | deb5d86b20c02312023959bae06b0fe651a4b2f4.1646653822.git.doebel@amazon.de (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [1/1] xen/x86: Livepatch: support patching CET-enhanced functions | expand |
Please discard this accidental copy of patch 2/2. On 07.03.22 12:53, Bjoern Doebel wrote: > Xen enabled CET for supporting architectures. The control flow aspect of > CET expects functions that can be called indirectly (i.e., via function > pointers) to start with an ENDBR64 instruction. Otherwise a control flow > exception is raised. > > This expectation breaks livepatching flows because we patch functions by > overwriting their first 5 bytes with a JMP + <offset>, thus breaking the > ENDBR64. We fix this by checking the start of a patched function for > being ENDBR64. In the positive case we move the livepatch JMP to start > behind the ENDBR64 instruction. > > To avoid having to guess the ENDBR64 offset again on patch reversal > (which might race with other mechanisms adding/removing ENDBR > dynamically), use the livepatch metadata to store the computed offset > along with the saved bytes of the overwritten function. > > Signed-off-by: Bjoern Doebel <doebel@amazon.de> > CC: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> > CC: Ross Lagerwall <ross.lagerwall@citrix.com> > ---- > Note that on top of livepatching functions, Xen supports an additional > mode where we can "remove" a function by overwriting it with NOPs. This > is only supported for functions up to 31 bytes in size and this patch > reduces this limit to 30 bytes. > --- > xen/arch/x86/livepatch.c | 63 +++++++++++++++++++++++++++++++++++----- > 1 file changed, 55 insertions(+), 8 deletions(-) > > diff --git a/xen/arch/x86/livepatch.c b/xen/arch/x86/livepatch.c > index 65530c1e57..da7611c01d 100644 > --- a/xen/arch/x86/livepatch.c > +++ b/xen/arch/x86/livepatch.c > @@ -14,11 +14,29 @@ > #include <xen/vm_event.h> > #include <xen/virtual_region.h> > > +#include <asm/endbr.h> > #include <asm/fixmap.h> > #include <asm/nmi.h> > #include <asm/livepatch.h> > #include <asm/setup.h> > > +/* > + * CET hotpatching support: We may have functions starting with an ENDBR64 > + * instruction that MUST remain the first instruction of the function, hence > + * we need to move any hotpatch trampoline further into the function. For that > + * we need to keep track of the patching offset used for any loaded hotpatch > + * (to avoid racing against other fixups adding/removing ENDBR64 or similar > + * instructions). > + * > + * We do so by making use of the existing opaque metadata area. We use its > + * first 4 bytes to track the offset into the function used for patching and > + * the remainder of the data to store overwritten code bytes. > + */ > +struct x86_livepatch_meta { > + uint8_t patch_offset; > + uint8_t instruction[LIVEPATCH_OPAQUE_SIZE - sizeof(uint8_t)]; > +}; > + > static bool has_active_waitqueue(const struct vm_event_domain *ved) > { > /* ved may be xzalloc()'d without INIT_LIST_HEAD() yet. */ > @@ -104,18 +122,36 @@ void noinline arch_livepatch_revive(void) > > int arch_livepatch_verify_func(const struct livepatch_func *func) > { > + BUILD_BUG_ON(sizeof(struct x86_livepatch_meta) != LIVEPATCH_OPAQUE_SIZE); > + > /* If NOPing.. */ > if ( !func->new_addr ) > { > + struct x86_livepatch_meta *lp; > + > + lp = (struct x86_livepatch_meta *)func->opaque; > /* Only do up to maximum amount we can put in the ->opaque. */ > - if ( func->new_size > sizeof(func->opaque) ) > + if ( func->new_size > sizeof(lp->instruction) ) > return -EOPNOTSUPP; > > if ( func->old_size < func->new_size ) > return -EINVAL; > } > - else if ( func->old_size < ARCH_PATCH_INSN_SIZE ) > - return -EINVAL; > + else > + { > + /* > + * Space needed now depends on whether the target function > + * starts with an ENDBR64 instruction. > + */ > + uint8_t needed; > + > + needed = ARCH_PATCH_INSN_SIZE; > + if ( is_endbr64(func->old_addr) ) > + needed += ENDBR64_LEN; > + > + if ( func->old_size < needed ) > + return -EINVAL; > + } > > return 0; > } > @@ -127,15 +163,21 @@ int arch_livepatch_verify_func(const struct livepatch_func *func) > void noinline arch_livepatch_apply(struct livepatch_func *func) > { > uint8_t *old_ptr; > - uint8_t insn[sizeof(func->opaque)]; > + struct x86_livepatch_meta *lp; > + uint8_t insn[sizeof(lp->instruction)]; > unsigned int len; > > + lp = (struct x86_livepatch_meta *)func->opaque; > + lp->patch_offset = 0; > old_ptr = func->old_addr; > len = livepatch_insn_len(func); > if ( !len ) > return; > > - memcpy(func->opaque, old_ptr, len); > + if ( is_endbr64(old_ptr) ) > + lp->patch_offset += ENDBR64_LEN; > + > + memcpy(lp->instruction, old_ptr + lp->patch_offset, len); > if ( func->new_addr ) > { > int32_t val; > @@ -143,14 +185,15 @@ void noinline arch_livepatch_apply(struct livepatch_func *func) > BUILD_BUG_ON(ARCH_PATCH_INSN_SIZE != (1 + sizeof(val))); > > insn[0] = 0xe9; /* Relative jump. */ > - val = func->new_addr - func->old_addr - ARCH_PATCH_INSN_SIZE; > + val = func->new_addr - (func->old_addr + lp->patch_offset > + + ARCH_PATCH_INSN_SIZE); > > memcpy(&insn[1], &val, sizeof(val)); > } > else > add_nops(insn, len); > > - memcpy(old_ptr, insn, len); > + memcpy(old_ptr + lp->patch_offset, insn, len); > } > > /* > @@ -159,7 +202,11 @@ void noinline arch_livepatch_apply(struct livepatch_func *func) > */ > void noinline arch_livepatch_revert(const struct livepatch_func *func) > { > - memcpy(func->old_addr, func->opaque, livepatch_insn_len(func)); > + struct x86_livepatch_meta *lp; > + > + lp = (struct x86_livepatch_meta *)func->opaque; > + > + memcpy(func->old_addr + lp->patch_offset, lp->instruction, livepatch_insn_len(func)); > } > > /* Amazon Development Center Germany GmbH Krausenstr. 38 10117 Berlin Geschaeftsfuehrung: Christian Schlaeger, Jonathan Weiss Eingetragen am Amtsgericht Charlottenburg unter HRB 149173 B Sitz: Berlin Ust-ID: DE 289 237 879
diff --git a/xen/arch/x86/livepatch.c b/xen/arch/x86/livepatch.c index 65530c1e57..da7611c01d 100644 --- a/xen/arch/x86/livepatch.c +++ b/xen/arch/x86/livepatch.c @@ -14,11 +14,29 @@ #include <xen/vm_event.h> #include <xen/virtual_region.h> +#include <asm/endbr.h> #include <asm/fixmap.h> #include <asm/nmi.h> #include <asm/livepatch.h> #include <asm/setup.h> +/* + * CET hotpatching support: We may have functions starting with an ENDBR64 + * instruction that MUST remain the first instruction of the function, hence + * we need to move any hotpatch trampoline further into the function. For that + * we need to keep track of the patching offset used for any loaded hotpatch + * (to avoid racing against other fixups adding/removing ENDBR64 or similar + * instructions). + * + * We do so by making use of the existing opaque metadata area. We use its + * first 4 bytes to track the offset into the function used for patching and + * the remainder of the data to store overwritten code bytes. + */ +struct x86_livepatch_meta { + uint8_t patch_offset; + uint8_t instruction[LIVEPATCH_OPAQUE_SIZE - sizeof(uint8_t)]; +}; + static bool has_active_waitqueue(const struct vm_event_domain *ved) { /* ved may be xzalloc()'d without INIT_LIST_HEAD() yet. */ @@ -104,18 +122,36 @@ void noinline arch_livepatch_revive(void) int arch_livepatch_verify_func(const struct livepatch_func *func) { + BUILD_BUG_ON(sizeof(struct x86_livepatch_meta) != LIVEPATCH_OPAQUE_SIZE); + /* If NOPing.. */ if ( !func->new_addr ) { + struct x86_livepatch_meta *lp; + + lp = (struct x86_livepatch_meta *)func->opaque; /* Only do up to maximum amount we can put in the ->opaque. */ - if ( func->new_size > sizeof(func->opaque) ) + if ( func->new_size > sizeof(lp->instruction) ) return -EOPNOTSUPP; if ( func->old_size < func->new_size ) return -EINVAL; } - else if ( func->old_size < ARCH_PATCH_INSN_SIZE ) - return -EINVAL; + else + { + /* + * Space needed now depends on whether the target function + * starts with an ENDBR64 instruction. + */ + uint8_t needed; + + needed = ARCH_PATCH_INSN_SIZE; + if ( is_endbr64(func->old_addr) ) + needed += ENDBR64_LEN; + + if ( func->old_size < needed ) + return -EINVAL; + } return 0; } @@ -127,15 +163,21 @@ int arch_livepatch_verify_func(const struct livepatch_func *func) void noinline arch_livepatch_apply(struct livepatch_func *func) { uint8_t *old_ptr; - uint8_t insn[sizeof(func->opaque)]; + struct x86_livepatch_meta *lp; + uint8_t insn[sizeof(lp->instruction)]; unsigned int len; + lp = (struct x86_livepatch_meta *)func->opaque; + lp->patch_offset = 0; old_ptr = func->old_addr; len = livepatch_insn_len(func); if ( !len ) return; - memcpy(func->opaque, old_ptr, len); + if ( is_endbr64(old_ptr) ) + lp->patch_offset += ENDBR64_LEN; + + memcpy(lp->instruction, old_ptr + lp->patch_offset, len); if ( func->new_addr ) { int32_t val; @@ -143,14 +185,15 @@ void noinline arch_livepatch_apply(struct livepatch_func *func) BUILD_BUG_ON(ARCH_PATCH_INSN_SIZE != (1 + sizeof(val))); insn[0] = 0xe9; /* Relative jump. */ - val = func->new_addr - func->old_addr - ARCH_PATCH_INSN_SIZE; + val = func->new_addr - (func->old_addr + lp->patch_offset + + ARCH_PATCH_INSN_SIZE); memcpy(&insn[1], &val, sizeof(val)); } else add_nops(insn, len); - memcpy(old_ptr, insn, len); + memcpy(old_ptr + lp->patch_offset, insn, len); } /* @@ -159,7 +202,11 @@ void noinline arch_livepatch_apply(struct livepatch_func *func) */ void noinline arch_livepatch_revert(const struct livepatch_func *func) { - memcpy(func->old_addr, func->opaque, livepatch_insn_len(func)); + struct x86_livepatch_meta *lp; + + lp = (struct x86_livepatch_meta *)func->opaque; + + memcpy(func->old_addr + lp->patch_offset, lp->instruction, livepatch_insn_len(func)); } /*
Xen enabled CET for supporting architectures. The control flow aspect of CET expects functions that can be called indirectly (i.e., via function pointers) to start with an ENDBR64 instruction. Otherwise a control flow exception is raised. This expectation breaks livepatching flows because we patch functions by overwriting their first 5 bytes with a JMP + <offset>, thus breaking the ENDBR64. We fix this by checking the start of a patched function for being ENDBR64. In the positive case we move the livepatch JMP to start behind the ENDBR64 instruction. To avoid having to guess the ENDBR64 offset again on patch reversal (which might race with other mechanisms adding/removing ENDBR dynamically), use the livepatch metadata to store the computed offset along with the saved bytes of the overwritten function. Signed-off-by: Bjoern Doebel <doebel@amazon.de> CC: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> CC: Ross Lagerwall <ross.lagerwall@citrix.com> ---- Note that on top of livepatching functions, Xen supports an additional mode where we can "remove" a function by overwriting it with NOPs. This is only supported for functions up to 31 bytes in size and this patch reduces this limit to 30 bytes. --- xen/arch/x86/livepatch.c | 63 +++++++++++++++++++++++++++++++++++----- 1 file changed, 55 insertions(+), 8 deletions(-)