On Mon, Jun 28, 2021 at 05:22:30PM +0100, Robin Murphy wrote: > From: Robin Murphy <robin.murphy@xxxxxxx> > Subject: [PATCH] arm64: Avoid premature usercopy failure > > Al reminds us that the usercopy API must only return complete failure > if absolutely nothing could be copied. Currently, if userspace does > something silly like giving us an unaligned pointer to Device memory, > or a size which overruns MTE tag bounds, we may fail to honour that > requirement when faulting on a multi-byte access even though a smaller > access could have succeeded. > > Add a mitigation to the fixup routines to fall back to a single-byte > copy if we faulted on a larger access before anything has been written > to the destination, to guarantee making *some* forward progress. We > needn't be too concerned about the overall performance since this should > only occur when callers are doing something a bit dodgy in the first > place. Particularly broken userspace might still be able to trick > generic_perform_write() into an infinite loop by targeting write() at > an mmap() of some read-only device register where the fault-in load > succeeds but any store synchronously aborts such that copy_to_user() is > genuinely unable to make progress, but, well, don't do that... > > Reported-by: Chen Huang <chenhuang5@xxxxxxxxxx> > Suggested-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx> > Signed-off-by: Robin Murphy <robin.murphy@xxxxxxx> Thanks Robin for putting this together. I'll write some MTE kselftests to check for regressions in the future. > diff --git a/arch/arm64/lib/copy_from_user.S b/arch/arm64/lib/copy_from_user.S > index 95cd62d67371..5b720a29a242 100644 > --- a/arch/arm64/lib/copy_from_user.S > +++ b/arch/arm64/lib/copy_from_user.S > @@ -29,7 +29,7 @@ > .endm > .macro ldrh1 reg, ptr, val > - user_ldst 9998f, ldtrh, \reg, \ptr, \val > + user_ldst 9997f, ldtrh, \reg, \ptr, \val > .endm > .macro strh1 reg, ptr, val > @@ -37,7 +37,7 @@ > .endm > .macro ldr1 reg, ptr, val > - user_ldst 9998f, ldtr, \reg, \ptr, \val > + user_ldst 9997f, ldtr, \reg, \ptr, \val > .endm > .macro str1 reg, ptr, val > @@ -45,7 +45,7 @@ > .endm > .macro ldp1 reg1, reg2, ptr, val > - user_ldp 9998f, \reg1, \reg2, \ptr, \val > + user_ldp 9997f, \reg1, \reg2, \ptr, \val > .endm > .macro stp1 reg1, reg2, ptr, val > @@ -53,8 +53,10 @@ > .endm > end .req x5 > +srcin .req x15 > SYM_FUNC_START(__arch_copy_from_user) > add end, x0, x2 > + mov srcin, x1 > #include "copy_template.S" > mov x0, #0 // Nothing to copy > ret > @@ -63,6 +65,12 @@ EXPORT_SYMBOL(__arch_copy_from_user) > .section .fixup,"ax" > .align 2 > +9997: cmp dst, dstin > + b.ne 9998f > + // Before being absolutely sure we couldn't copy anything, try harder > +USER(9998f, ldtrb tmp1w, [srcin]) > + strb tmp1w, [dstin] > + add dst, dstin, #1 Nitpick: can we do just strb tmb1w, [dst], #1? It matches the strb1 macro in this file. Either way, it looks fine to me. Reviewed-by: Catalin Marinas <catalin.marinas@xxxxxxx>
