encoding/pem: Decode method uses extensive CPU when providing large data

[naveensrinivasan]

What version of Go are you using (go version)?

$ go version
17.5

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
go env
GO111MODULE=""
GOARCH="amd64"
GOBIN="/home/sammy/go/bin"
GOCACHE="/home/sammy/.cache/go-build"
GOENV="/home/sammy/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/sammy/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/sammy/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/nix/store/i8x0bv8v69j8c2njymgs4cl3albqc20h-go-1.17.5/share/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/nix/store/i8x0bv8v69j8c2njymgs4cl3albqc20h-go-1.17.5/share/go/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.17.5"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/dev/null"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/run/user/1000/go-build3984378223=/tmp/go-build -gno-record-gcc-switches"

What did you do?

As part of fuzz testing sigstore/sigstore#188 using oss-fuzz we ran into an issue where the pem.Decode is consuming extensive CPU and here is the test case for it https://github.com/sigstore/sigstore/files/7676542/clusterfuzz-testcase-minimized-FuzzUnmarshalCertificatesFromPEM-4710187355734016.zip

What did you expect to see?

The pem.Decode finishing sooner than it consumes large amount of CPU

What did you see instead?

It being slow

[cherrymui]

cc @rsc