Re: [BUG] arm64: an infinite loop in generic_perform_write()
From: Catalin Marinas <catalin.marinas@arm.com>
Date: 2021-06-29 08:31:01
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linux-arm-kernel, lkml
On Mon, Jun 28, 2021 at 05:22:30PM +0100, Robin Murphy wrote:
From: Robin Murphy <robin.murphy@arm.com> 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 <redacted> Suggested-by: Al Viro <viro@zeniv.linux.org.uk> Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Thanks Robin for putting this together. I'll write some MTE kselftests to check for regressions in the future.
quoted hunk ↗ jump to hunk
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@arm.com>